Trying to gather tulling

Here's a manual for your machine:

ELNA eXcellence 730 Instruction Manual

If your machine does not have a "basting" stitch selection, you make your own basting stitch by:

* Setting the stitch length to the maximum
* Loosen the upper tension so the bobbin thread lies flat under the fabric

Then to gather or remove the basting stitch, it is easy to pull on the bobbin thread.

Might be a good idea to check out sewing "how to" books at the library.

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Are your feed dogs down? If not check your stitch length or possibly it is set to tack stitch. What material are you trying to sew? If it is lightweight like tulle you will have to set your feed dogs. Check your manual on how to do this.

check to make sure that around the tension dial there is no lint or dust
the upper tension spring could be bad
a sewing machine repair shop could tell about the tension spring

Sounds like your machine is off timing, When the machine is properly timed, you should see the hook passing behind the eye of the needle when the needle is on it's upward stroke. If this is not correct the machine will not pick up the bobbin thread. You may have to take it in for adjusting, unless your a bit machnical.

Try going to your Bernina dealer and see if they have any suggestions because they can evaluate if it is serious or just a simple adjustment. You have a great brand and you do not need to have it messed up by not having someone try to fix it when they do not have the right tools or know how. It may be due for a simple tuneup.

Try to make two parallel lines of straight sewing on a big stitch, then pull both ends together. this gives the best gather. Tighening the tension to make the thread pull doesn't work as well, although its OK

Yes, if there is an iron-on adhesive being stitched over.

A bead of Sewer's aid along the seam line in advance of stitching will keep the needle clean - but, with tulle, it may not adhere.

Try it on the top thread by running a bead of SA down the side of the spool to lightly coat the thread.

Try setting the differential feed to it's lowest setting.

Test with scraps of what you will actually be stitching.

You may need to purchase a Gathering Attachment
Part No. 200 217 101

BUT be sure it is suitable for your model first
(I am unsure which one you have)

Thread tensions are set at manufacture to all = 3 for standard work, your manual is generally helpful for many answers.

The "Janome Overlocker Sewing Guide" is a very detailed publication and has this to say about Gathering without any additional attachment.