In our mother's time, thread was thread. It was all the same.
Nowadays, there are several weights of thread, and it's made of all kinds of combinations of things. So you have to treat each spool of thread as if you are starting new.
Make sure your pressor foot is up when you thread, otherwise the thread may not go into the tension assembly properly.
Adjust the top tension, in small increments until it's right.
If you can't get good stitches by adjusting the top tension, you then try adjusting the bottom tension - just make sure you use tiny adjustments - don't go more that 1/12 th of a turn at a time. (like 5 min on a clock)
Often the problem is using different threads in the top and the bottom. Try using the same brand and weight on both.
Experiment!