Just some thoughts...it sounds like you have had some of the common things checked- filter, pump, hoses, etc.
Since you plug your cleaner into the skimmer directly (from what I have read), you are subjecting your suction line to clogs. All the debris that your vac picks up can bunch up in the pipe and cause a blockage, so when you try to run your vac- the potential clog and the extra head pressure from the hose strains the pump and reduces suction. One way we look for blockage clues is with your vac hose plugged in and the pump on...observe the pump and shut it off. If water seems to rush backwards...then that is a blockage clue. The pump is trying to pull the water but can't, so when the pump is shut off, the energy is sent backwards...make sense? We have several techniques for clearing clogs...mostly we use a tank of compressed air, special plugs and "blast" air from the pump backwards (or from the skimmer, depending on situation). We also use special plugs to "plunge" the pipe. Do not snake the line, this can be very damaging, unless the person snaking has experience.
If you do find that the line was clogged and you fix it, alter your cleaning methods by adding either a "vac-mate" type apparatus to the skimmer or inserting a "leaf catcher" in-line with your cleaner hose. If not and it is determined that the problem is not a blockage, maybe its time to call a leak specialist to diagnose the line. First thing they should do is pressure test it- to ensure no leaks are present, and then if they are knowledgeable, they can de-block the line.
Keep us posted, maybe we have more advice. Good Luck!