I think you should try ramping up with low reps. In the past i barely dipped anything above body weight so ramped up with sets of 3-5 that were still explosive reps, within a couple of months i was hitting out sets of 12 with 45kg round my waist.
Also, when in the workout do you do the dips, if after other exercises maybe your already too fatigued.
It's not always easy to gauge improvement at an exercise where the load is constant (your bodyweight). As such, I've never really focused on getting better AT dips, but at getting more OUT OF doing them. Instead of even bothering doing them at the beginning of my triceps training and worrying about the reps going up, I use them as a finishing exercise, knowing that they take a hell of a lot out of me, so as long as I but my ass, leaving nothing in the tank, I leave the gym with a pump from hell, knowing I 'got the job done'.
This sounds dumb i dont even know if it is what helped me, but just getting my triceps stronger in general trough CGBP helped my dips a lot, i usually dont do the free weight ones because i feel shouklder strain but i do the machine now with 10 plates for 8 reps, if someone doesnt believe i could post a video next week when i do arms:)
Try the force spectrum ramping Thibs talks about. You can find the write up on I,BB if you like. I used it and found it to be a pretty legit training technique. Definitely keeping it in my arsenal as a plateau buster. Hasn't failed me once yet.
If you go ahead with it, let me know how it goes. Give the technique a good 6 to 8 week trial though. I actually gained some significant strength with it.
Ive noticed this. Previously i would use dips as a warm up and would seriously screw up my left shoulder/rotator cuff if i went to failure or pushed too hard. Now i do them at the end after my main work there have been few if any issues, and my dip numbers are going up.