Read through these sites for excellent information on sewing machine problems. Superior Threads has a particularly informative Education section worth your time.

https://sewguide.com/troubleshoot-se...hine-problems/
https://www.superiorthreads.com/educ...waAvz_EALw_wcB
http://shesasewingmachinemechanic.bl...-to-check.html
https://www.abcsewingmachine.com/blo...d-fix-yourself
https://www.superiorthreads.com/education