Melvin,

Is the whole line sinking except the tip or is the tip sinking? My guess is the latter.

If it's a floating line, a problem could be that it's soaking up water. It might be cracked, permanently drenched or simply dirty. Try cleaning it, drying it thoroughly and treating it with a line conditioner. If it still sinks, try some floatant like Mucelin or silicone on the tip, and if that doesn't help, you simply have a sink tip line. Use it as such and buy a new floating line.

Martin