A couple of things.
Do you know about the Nitrogen Cycle?
Do you have Test Kits for your tank for ammonia, Nitrites, Nitrates and ph?
If yes, test your water. If not, either buy the kits or have your water tested at the local pet store. They will usually test it for free if you bring a sample in a zip lock baggy.
Also, I am not sure you are aware of this or not, but sailfin plecos reach lengths in excess of a foot. Usually 16 inches is around the average size for a full grown sailfin. These fish require a tank size of 125 Gallons to live their life in, otherwise they will stunt and die. Do not treat this fish for Ich. Plecos do not usually get ich for one, and for 2, ich looks like a fish has been sprinkled with salt, not cloudy patches.
How fast and how many fish did you add to your tank?
If you added a whole bunch of fish at the same time, it is possible the ammonia and nitrites skyrocketed and are causing your fish to have a toxic reaction.
Plecos are virtually impossible to kill, they are very hardy and adaptable.
If you are having troubles with your pleco, my guess is there is something in your water that is causing harm.
How often do you do water changes? What is the tank temperature?
How many of each type of fish is in the tank?
Do you suck the gravel when you do water changes?
What type of filter do you have and when did you last change the media?
If you can provide some answers for me, I can probably help you sort out whats wrong with your fish!