If the steps suggested by @DWGKNZ don't work you should consider taking it apart and inspect the Piston seals. Over time the piston seals can dry up and get gummed up with a little dirt and what looks like limestone.
Overhauling most hydraulic brake systems is doable by the home mechanic and Avid sells a parts kit for the Juicy 7's.
After I rebuilt mine I noticed the pistons still didn't retract evenly. One side would move more then the other. The reason for this is the friction between the piston and the piston seal. Dot fluid will help lubricate the retraction movement but that alone may not be enough. Ideally as part of the overhaul you should use some form of silicon grease. Avid sells a compound called DOT GREASE that is specifically designed for piston seals and O rings.
You also need a bleed kit if your are going to rebuild.
- Take the pads off and clean the area with brake cleaner.
- Pump the lever to push the pistons out a 4-5 mm and dab dot grease around the piston edge with a cotton wool bud or very small paint brush.
- Push the piston back in and repeat the process a few times, forcing the grease into the piston seal surface.
Eventually it should be nicely lubricated and pistons should retract quickly and evenly.
Obviously it would be easier to just buy a new set of brakes but I personally learned a great deal by doing it myself and found the whole process very rewarding when you finally fix it.