How old is the skimmer? How long has the skimmer been running for and what size tank do you have?
If the skimmer is new, then it needs to break in. During the break in period (at least 3 weeks) the skimmer will act erratically and will be very inconsistent. Some days it will not collect at all and some days it will collect nicely. If you force the skimmer to always collect during this period (closing the valve or placing the skimmer in deeper water) you will run into overflowing issues. The skimmer needs sufficient area for the water and air to properly mix before overflowing to the collection cup. If not enough space is given, the skimmer will still work but will end up overflowing more often than not.
We recommend placing the skimmer in 6.5" of water from the get go and opening the gate valve all the way. Leave the skimmer as is for the first 3 weeks and after the skimmer is broken in, then you can go ahead and start tuning it by closing the gate valve and if needed, reducing the speed of the pump.
Once the skimmer is broken in, here is a quick read on how to properly dial the skimmer...
http://coralvue.com/support/317511-How-do-you-dial-in-a-protein-skimmer
Let us know if you have any other questions...
Regards,
Carlos