When reels get used lots particularly cheaper ones the bail starts to get sticky and doesnt close tight, like it did when new. That will do it. I got one that eventually no matter how much i oiled it and stretched the spring , it did that.
It particularly didnt like stiffer lines like flouro after the bail started acting up. Try using Tri-Lene XL maybe.
I am assuming its a spinning reel. You said light rod combo so. If its baitcaster disregard.