Upper thread getting tangled in the bobbin

try increasing/decreasing the needle thread tension for the added material thickness
use scrap material until you get it right

First, install a brand new needle. A bent/damaged needle will frequently skip stitches. Also, be sure you are using the correct needle for the fabric and thread you are using, ie do not use a ball-point needle on woven fabric.

sewing machine skip stitches Google Search

If you've tried all the suggestions and it is still skipping stitches, the machine may be out of time. Search for sewing machine timing or take it for service.

use a couple of pieces of waste material the same as the sweat pants material and sew about an inch long
adjust the needle tension and do it again
repeat the exercise until the top and bottom thread joint at the middle of the two pieces of the material
tension settings on the machine are a loose guide as the thread thickness, material thickness and needle size all have a bearing in the tensions of the thread
so practice and adjust prior to sewing is the way to go

A transcript from the Elna SU manual reads as follows, and the manual can be viewed on line at http://sewingonline.co.uk/instructions/elnasp/

First make a few trial runs with a scrap of material exactly as the buttonhole will be stitched, with the machine adjusted as you intend to use it.
1. Use the embroidery and buttonhole foot. It can be chnged without a screwdriver by using the fixation screw.- With the stitch width knob at "0", turn the selector to zigzag "1".
2. De-center the needle to the left.
3. Set the stitch width knob at 2, with the solid mark on top.
4. Set the stitch length dial between 1/2 and 1/4.
5. a) Mark the desired length on the material.
b) Sew the first side of the buttonhole. Stop with the needle in the material and the mark (right).
c) Raise the foot and pivot the material round the needle. Lower the foot and raise the needle.
6. a) Set the stitch width knob at 4.
b) Sew a few stitches. Raise the needle.
7. a) Set the stitch width knob at 2.
b) Sew the second side and stop just short of the length of the first. Raise the needle.
8. a) Set the stitch width knob at 4.
b) Finish off the buttonhole by sewing a few stitches. Raise the needle.
9. a) Set the stitch width knob at "0".
b) Sew a few stitches to fasten the threads, holding the material back by hand.
10. Open with the buttonhole knife, which you will find in the accessory box.

I think you are saying no stitch is formed also referred to as the needle not catching thread. Below offers a variety of diagnoses and solutions which are straight forward.

Sewing Machine Needle Not Catching Thread

Below is general advice quoted from 6 common sewing machine problems solved:

Skipped stitches are usually due to a bad needle. The needle may have become damaged or bent by sewing material too thick for the needle, forcing material through the feed dog, or hitting a straight pin. If the problem seems to be consistently reoccurring, it may be due to forcing the fabric. Sewers should allow the feed dog to pull the fabric and only use their hands to guide the fabric through the feed. When sewing knitted fabrics, using a stretch needle can help prevent skipped stitches.

Below is a video showing how thread is hooked into a fabric:

Sewing Machine Anatomy How Stitch is Made

We can sew about 2 to 4 inches then the thread that goes through the needle snaps off & when you examine the back of the material the bottom stitch is loose. What is wrong?

have you checked the needle to make sure it's in correctly--flat side of needle goes to the back of the machine

correct needle insertion is mandatory for the proper use in your sewing machine. Raise the needle bar to the highest point, then loosen the needle clamp screw to remove the old needle.
place the new needle in the needle clamp with the flat side of the needle facing the back (or if using a round needle the groove in the needle should be facing away from the shaft.
push the needle all the way up to the stop, or as far as it will go, then tighten the needle clamp screw.

how to decide which needle is correct to sew with, it depends on the fabric & thread being used for the project.
like for example, I've been sewing for over 30 years, I'm sewing a stretchy fabric & regular needles won't work because they skip stitches & will pull the fabric thus damaging the fabric, the needle I have to use is a singer universal ball point needles, & I have no problem with the stitches they come out fine.

The stitch length dial needs to be set to a lower number (number of stitches per inch) to give you longer stitches. The tensions do not regulate the stitch length, the feed dogs regulate the distance the material is advanced between stitches. Also, make sure the pressure foot is applying enough pressure to keep the material in firm enough contact with the feed dogs.
<Ray>

hi taylor.
not sure if this link will help you to find out why,but have a look.
Kenmore Ultra Stitch 10 sewing machine needle keeps breaking Google Search

http://www.sewusa.com/Sewing_Machine_Instruction_Manuals.htm