Just to continue the thought ....
I've never had a fly line of any brand that that the tip end would not be sinking after some use. The solution as already posted is (#1) line cleaning (#2) line dressing. I know there are many, many fly line dressings available today and no doubt many variations used my members of this board, but let me say that I came across one some time back (I think at a fly shop) that has worked so well ... brand name is Accardo - which I know will cure the fly line tip sinking.
I keep my fly lines cleaned with line dressing applied, and I get long life out of them.