What type of vac do you use? Barracuda style?
Are you getting a good seal at the skimmer connection?
Possibly something stuck in/blocking your vac head?
Could also be that you have a small air leak between the skimmer and pump, teh extra suction req'd to drive the vac can easily cause a small air leak to become more prominent to the point of inducing enough air to cause you problems with suction.