Excellent post EB (as usual)
The only thing he didn't mention is the fabric being sewn, this too can contribute or combine with other factors in a tight or loose stitch. The looser the weave, the longer your stitches need to be to grab enough fibers to hold the two pieces together. The back stitch Char recommended is especially valuable in these cases. Felt isn't woven, so too tight a stitch can actually tear through the fibers quite easily.
If you want to post pictures we can help you evaluate them
NOTE: not all threads degrade at the same rate, it depends on it's fiber content, how it was made, and how it was stored. Test each questionable one by cutting off a 12" section then wrapping the ends around your fingers like you are going to floss, pull it gently first, then taut with a snap, to see if it breaks. If the gentle pressure breaks it, it's trash. If gentle pressure is ok but the snap breaks it, it's really only suitable for decorative uses (embroidery or top stitching).
I threw away the Walmart thread after the first time I used it, it's awful stuff. I've thrown away thread spools from the 80's that degraded, but still have some nylon/ cotton blend button thread from the 50's that I could put my full body weight on because it's that tough.