Thread keeps on bunching up on the bottom side of the fabric

SOURCE: the lower stitch (under the fabric) is using too much thread

Check everywhere to get lint plugs out of the way and change your needle. Make sure it's in correctly. And, make sure you're threading correctly...don't forget the take-up hook.

SOURCE: Sewing machine problem

I just solved this problem with much persistence and a few possible causes. Mkae sure the thread is good quality and cant shred, at least until problem is fiwed. Unscrew plate behind bibbin and check if there is any caught thread, this happens after bottom thread has been looping. Remove any and replace, wind down the needle to check it goes in the centre of hole.
And probably the most important fix it; the tension;
Have your tension set to zero, thread the machine, and when the foot is DOWN, make sure it is very hard to pull the thread. It should be easy to pull the thread when the foot is UP. If the thread is easy both ways it has not caught correctly in the tension plates, thread it again (and again) until this happens. It shouldn't be possible to thread the machine without it going in the right place, but it is. And make sure when you rethread that the foot is UP until you thread the needle-
Another problem maker I also had to fix; ever so slightly bent needle.

SOURCE: Bicor vx1005 ...thread keeps bunching

Try a different bobbin. Look for one that is a little narrower. I had the same problem and changing to a narrower bobbin fixed it.

SOURCE: Looping thread bottom side of fabric

That means you have not routed the bobbin thread properly, or bobbin has not been filled properly. Could also mean your tension is incorrect.

SOURCE: the thread keeps bunching up

Totally rethread and put in a fresh needle. It should work wonders, if not go to brother-usa.com and troubleshoot