I asked a master lifter I train with about improving shoulder flexibility...I pasted his e-mail response below. Not sure if you're doing these already, but hope this helps.
As far as shoulder flexibility for racking cleans goes there are many stretches that are effective. Your shoulder routine should include:
1. Dislocates with a broom stick.
2. Rack stretches. Lock a bar into the power rack and have someone apply a steady, GENTLE upward stretch on your elbows. Lenny Bacino can help you with this if you are there on Wednesday. Keep the pressure on for 20 seconds, relax for 10 seconds then repeat a few times.
3. Practice power cleans with a light weight 20, 30 or 40 kilos. Do lots of reps and concentrate on whipping the elbows and keeping the shoulder girdle relaxed. Do the reps from the floor and try to do 6-10 reps a minute for 3 minutes or more. You can rest in the start position for a few seconds between reps. These should make you sweat but not tire you out.
4. If you can rack a power clean properly but can't rack a squat clean very well then your flexibility challenge is probably in the back -- maybe the rhomboids or the lower spinal erectors.
5. And of course, don't neglect the triceps. They need to be stretched too. Pull your flat hand (palm up) back and down on top of your deltoids. Hold the position for 20 seconds, relax for 10, repeat.
See you at the gym!