not all invisible threads are created equal- some stretch & break easily- if using a high-speed machine- some are (pokey) like fishing line-
some melt if touched with an iron- or become brittle and break if placed in a hot dryer.
so choose a good quality- recommended one (monofilament- not nylon) bobbin thread to match your background & slow down - some are easier to work with than others- if you order your invisible thread read the reviews & make sure you are getting what you want- if purchasing locally - from the lqs- ask what they recommend & why-