Could be a number of things:
-Line is worn out, degrading
*solution: run your hand along the line to feel the line to ensure there's no rubbing damage
-Old line, usually after fishing each year, you should replace the line. That's what I was taught anyways and it hasn't failed me yet. A new roll of 250 yards would be good for 2 years or two replacements. Costs about 20 bucks, so can't go wrong here.
-Sometimes fish have a last wind dash when they see you or sense they're close to shore, then muster up as much energy as possible and dash off. Especially when you're landing it in by hand, not a net. Hence the benefit of having a net and someone else to help you net the fish. This can be controversial, some people say that playing the fish until it turns up by the side is ok to reduce this last minute dash. Personally I don't feel this is good for the fishery overall since you have to really make sure that the fish has their energy back to release the fish, something that needs to be treated with respect, especially if you cannot retain the fish and it's a protected species.
-or you're right, your knot is not working out as well. Experiment, test it by hand. It shouldn't break when you're pulling it in opposite directions. Some people practice the habit of testing the knot every 5 casts or so. Can't hurt to have this habit and reduce the $$ for your lost hooks, spinners, spoons, bait etc etc.