I have a brother ls-2125 sewing machine. the upper thread is not catching the bobbin thread properly. help please

1. Tension
   • Your sewing machine tension controls the rate at which the thread is fed through either the upper section of your machine or through the bobbin case. If your upper or bobbin threads aren't threading properly, sometimes adjusting the tension will correct this problem.
     
   Timing
   • Your sewing machine works by passing the upper thread through the needle of the sewing machine at precisely the same time the bobbin brings its thread to its uppermost position. If either the needle or the bobbin thread reach these positions too soon or too late, the threads won't interact and create the knots that make the seam. You can throw off your timing by sewing too many layers of fabric or hitting a pin with your needle. See you sewing machine repair facility to correct any timing issues.
     
   Thread
   • Thread can get snagged or caught in the upper section of your machine or on lint pieces in the lower bobbin section. If your threads aren't catching properly, you may be able to fix it by unthreading and rethreading both your upper thread and bobbin threads.
     

Have you tried to change the needle? if the needle is not perfect, the bobbin won't catch the upper thread correctly. There is only about 0.1 mm of space to do that, so even a needle that looks perfect coud be very slightly bent and give problems while stitching. First of all , in your place I would try to substitute the needle.
Then, are you using one of the original plastic bobbins? I once bought sone cheap "universal" bobbins that were only vey slightly smaller than brother ones (VERY very slightly smaller),..Well, the machine went crazy and I had a lot of problems with sipped stitches until I read on the internet NOT to use "universal" bobbins, as bobbins look the same but are not made exactly the same way and they must be EXACTLY those made for your machine...I bought some bobbins that were made especially for this sewing machine, and it never skipped a stitch again! (It's possible that other sewing machines work wel with universal bobbins, but not this one, at least not with the universal bobbins I had bought!)