Sewng machine fails to loop on bottom bobbin

There are several possible causes for this problem.

Simplest first.

1) The machine has sat in the closet for three or more months, and the oil has ceased. The fine oil requried for these precision machines does run away, and/or cease up into a grease like paste, causing the parts that move in precise harmony, to be not so precise.
The cure for this, is to remove all thread and fabric, and run the machine full speed for about two to five minutes. This heats up the parts and loosens up ceased parts. A good re application of fresh oil helps too.
NOTE: Check the needle is Straight by turning the machine a couple of turns by hand and seeing that the needle dosn't hit any metal parts. A breaking needle can cause dammage to the machine or user in when operating at high speeds.

2) Tension dials have been knocked out of balance. There is a numbered tension dial on the front of the machine, somewhere along the threads path from spool to needle. if this is too tight, the thread will not leave a loop big enough to catch the bottom thread. Be sure this is set to about 3, usually. It should be adjusted while stitching, when you get to that point.
On the bottom thread, on the Bobbin Holder piece that comes out of the bottom, there is a small screw that adjusts tention of the thread comming out. This screw should not be tight, but tight enough. Completely by tactile intuition, this screw should be adjusted to allow the thread to slide out of the bobbin holder smoothly. It must have some tension to pull the thread from it, but not so much that it is noticeable, or take much of a grip on the thread to pull it out. (difficult to describe)
These two tensioners must balance so the threads, cross paths, but also pull the stitched snugly into the fabric being sewed.

3) Bent Needle, this is a problem if the fabric being sewed has pulled the needle off it's center. If it is not perfectly straight, don't use it.

4) Cheap thread can be a problem. If the thread is frayed, or fuzzy looking, it may be that it will not work with as fine a needle you are using. Needles have various sizes and sized holes to feed the thread through, a fine needle needs fine thread, and coarse thread needs a larger needle.

These are the primary causes of the threads not looping together properly. I hope this helps.