Singer 301 tension
 

I usually fill several bobbins before starting to sew. I then test the tension. Once I have it correct, I start sewing.

However, EVERY TIME I change bobbins on my Singer 301 I have to play with the tension - even if the top thread and the bobbin both contain exactly the same thread as the one that just ran out AND the fabric is exactly the same.

It is really frustrating.

Recently I spent over an hour playing with the upper and lower tensions. Many times the tension 'looks' good but if you tug on one of the thread tails the thread pulls right out of the fabric.

Are you winding the bobbins per what the manual shows? Tension is 50% top but you also have the bobbin case tension adding 50% to the equation. Have you checked the bobbin case tension?

Yes. Since I have several machines, I pull out the manual every time I change the bobbin or top thread or wind a bobbin. I have adjusted both the top and the bottom tensions.

possibly need to clean lint out of upper tension discs and/or under bobbin case leaf spring? soyndsxlike maybe one or both are light...

Some times you have to raise the foot before you put the thread through