how's your water?
I know kind of silly question, but could be your water vs. the salon's. If you have lots of minerals (hard water) it's almost impossible to use Deva products. I had the same issue (about $200 & 6 months waste) till I figured it out.
A great sub for Deva gels (IMO) is flaxseed gel, it's easily and cheaply made at home or Bottecelli Babe makes her own line (which folks rave about) available in Etsy.com.