I've never had a floating line that didn't eventually start to lose its floating gumption at the tip end.... sometimes straight out of the box and an hour or so's fishing, others after a couple of years of stiff use.
No rhyme or reason to it.... just a combination of plastic surface eventually breaking up under flexing and casting stresses and letting water in to the structure of the line, especially if the line has a braid core. Plus the thinner tip of the line has less %ge floating material round the core to bouy it up than is the case further back down the line on the thicker bulk stock.
Using a line floatant or other water repellent will work, some again better than others. Best applied after very very thorough drying out of the line (I tend to string it along my washing line up and down).
But I have never really ever 'cured' a line this way once it had started the sinking blues.
My alternative and simpler strategy is (a) relegate it to use it as a sink tip line and (b) buy a new floater - an expense and new toy you can now justify to both yourself and the other 'arf
Mind you, I haven't ever really had much of a problem with just a wee bit of the line sinking, specially for sub-surface nymphs etc. But if you have three or four feet or more badly affected and dragging it, the leader and your floating fly down under then it is usually time to say goodbye to that line.....