Is it fabric string? I might know the kind you’re talking about, its kind of a cheap plastic one with some string wrapped around it and it unravels over time because the strings aren’t like woven together and it has like a sparkly string in it? I think. I’ve used that kind for a few singles and they loosen over time. This one is the best I’ve used, 1 spool lasts a really long time and 3 square knots to tie it off with do the trick and it shouldn’t unravel.
Amazon! Bingcute 1.0MM Rainbow Elastic Cord Beading Threads Stretch String Fabric Crafting Cords for Jewelry Making, 100 Yard (Rainbow) https://www.amazon.com/dp/B07W2XJC63/ref=cm_sw_r_cp_api_glt_fabc_KQ20NCVKC3NSFZFD7STJ