What is sounds like is that you are getting too much water through your filter. If you overload the capacity of the sodium sulfate, some of the salt will dissolve into your extract and pull it through to the evaporation stage. You will notice your extract is very cloudy when this happens. There are a few things you can do to solve this problem. The first is to leave vacuum on your SPE filters for a longer period of time. You need to make sure you have pulled as much residual moisture out of the filters as possible. Be sure you have adequate vacuum for the number of samples you are extracting and leave the vacuum on the samples for 10-15 minutes or so after all the free liquid has pulled through. Then once you have added the hexane for extraction, if possible try to slow down the rate that the solvent is going through the sodium sulfate column. I don't know exactly how the Xenosep system is set up, but if there is a stopcock or valve of some sort, just open it partway so that the sodium sulfate has a chance to absorb the moisture instead of being absorbed by it. Also, you want to make sure the sodium sulfate is kept as dry as possible. Don't leave the bag open to the air, especially if you are in a humid environment.

I agree with everything David says here.  I looked up what the Xenosep device looks like and one thing I would definitely keep an eye on would be your vacuum when pulling the solvent through the sodium sulfate cartridge.  If you pull too much of a vacuum your hexane will go through the cartridge too quickly leading to water and/or sodium sulfate getting through.  That is definitely a critical step in any extraction device so start there.