Bobbin thread won't engage.

The manual may be helpful:

Brother CS 8072 Operation Manual

be sure to ALWAYS RAISE the presser foot when threading the top thread.

How to Thread Brother CS 8072 Sewing Machine eHow

http://support.brother.com/g/b/faqend.aspx?c=us_ot&lang=en&prod=hf_cs8072eus&faqid=faqh00000485_000

try using a longer needle, or lowering the one you have now by loosening the needle screw and dropping the needle down some

try changing the needle
try a different bobbin
try turning the bobbin the opposite direction
make sure the bobbin is inserted correctly into the bobbin case
there could be something wrong with the hook and hook race
and the machine may need some adjustment

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.
     

Check the bobbin tension and that the thread is tight around the bobbin. Make sure that the bobbin is in the right way around - the bobbin thread should double back on itself coming out of the bobbin holder.

Have no idea what brand of machine you are using.

If this is a rotary hook bobbin (side or front insert; metal bobbin case),
there should be a lever on the back of the bobbin case.

Raise the needle to it's highest point and then lift and hold the lever of the bobbin case open as you insert it into the shuttle.

If there is an arm on the bobbin case, it should be pointing up toward the machine.

When the bobbin case clicks into place, it should work as designed.

machine needs to be timed,,usually a 30-50 repair, could try dropping a needle just a tad bit to see if it catches

It sounds like you are not threading the machine properly. The best way is to make sure the presser foot is in the up position. This keeps the tension disc open and the thread slides into the tension easier. Also make sure you have the bobbin in and threaded correctly into the bobbin case.

You may have the bobbin case in wrong.  I have this machine, and I know that it is really easy to put the bobbin case in wrong.  Try taking it out and making sure that the little stop is in the front and it is seated down in there and able to wiggle back and forth.  My experience with this machine, until the other day when my cat knocked mine off the table and knocked something out of alignment, is that it just works.  Any time mine hasn't worked, it has either been dirty, needing oil, threaded wrong or the bobbin case wasn't seated right.  In other words, it has always been my fault.

Had same problem with another Machine...I also have a cs 8072. it jumped time. any sewing machine shop can fix it in unser an hour for around $20-$30. Good Luck!!