even when you (just bought) a spool of thread- it can be 'OLD' stores don't always rotate their stock- what is new to you may have sat in a back room for years.
i have bought $15 specialty threads from shows only to get it home & find it to be (bad thread) if you can take a length of the thread between your 2 hands-give it a tug and it breaks---it's old, or has been stored improperly- is not any good for stitching...doesn't matter if it's C&C, YLI, Mettler...happens to them all- brand has nothing to do with it.
i've learned over the years- when i open a new spool of thread the first thing i do is a 'tug-test'- if it breaks i don't bother threading my machine with it- i already know it's going to cause problems- i mark it & use it for basting- or some other purpose that isn't going to matter- saves me alot of frustration.