There can be several causes to your problem:
Rethread the machine top and bottom
using the correct size AND type of needle for your project
Using the correct needle manufacturer-Schmetz
Old thread
Thread in bobbin is not compatible with thread in needle
Proper tension in needle and proper tension in bobbin
Machine needs to be cleaned under the needle plate
Thread not seated in tension discs-Tension discs may need to be cleaned
Or your machine is out of time
First I would give the machine a complete cleaning, under the needle plate, in the bobbin area, and the tension discs
Put a small drop of oil in the bobbin area or wipe the bobbin area with oil on a Q-tip
Change the needle and make sure new needle is inserted properly-Flat side of shank is toward the back of the machine
Change your thread
If nothing else works It's time for the machine to have a "spa day" with a tech.