i would bet money one of your kegs has a small leak, i bought all my kegs off ebay and all where pressure tested, and shipped under pressure, so when i got them first thing i did was bleed off the CO2. well fast forward a few weeks later and my 10# co2 tank is empty after a month and around 2 kegs of beer. and one job of xfering beer from carboy > keg.
so i leak check and double leak check all of my kegs and find my poppits arent sealing completly and one of my check valves leak if you twist it just the wrong way when pulling it. so i stoped being cheap bought some keg lube, rebuilt all but 1 of my kegs (ran out of packings for it) and havent had a problem sense.
another thing i do is shut off the gas going to the kegs when not in constant use. i have 2 lagers that i will open the gas valve ever couple weeks just to make sure they are holding pressure or havent absorbed all the CO2 if i hear the keg fill up then i know the beer absorbed a small amount of CO2 if the keg is flat and has NO pressure it has a leak.
just my experece with kegs from ebay, side note, stuff shrinks when its cold think about jumping in a cold shower, O rings do the same thing when they are cold so purchase some keg lube itll soften those O rings up and you will get a better seal.
oh one more thing are you sealing your kegs after you fill them? i normally place around 30psi just to get the lid seated. then bleed about 10 psi off and let the keg sit with no gas going to it so the beer will absorb the CO2.