Hit a road block with the wound vac?

I have a client who arrived at my facilty with a stage 4 wound to his ishium. After a few weeks of the wet to dry dressings he arrived from the hospital with, we decided to try the wound vac. It has healing well up until a point. We are now having more trouble than ever keeping it from sidding off of the wound and interupting the treatment.
We have had and OOB schedule of only 2 hrs a day to keep him off the area. He had previosly been on a hoyar lift. Now though, he is stronger and able to get out of bed with 2 assist. Still is on the same OOB schedule of 2 hrs. However, NOW the vac is slipping off the wound on a much more frequent basis, and due to the site, it ias impossible to reenforce it. The sponge at this point is already out of the wound and pulled up. Also, I have noticed that there is no longer grainage in the cannister. Let me try to explain what we have been doing to apply the vac.
The area is irrigated with N/S. Dryed. Skin prep is applied around the wound and up the entire area where the bridge will sit. Because of the moistness of the area ( the client is also incont) I have been putting a small piece of duoderm near the peri area ( the very thin duoderm), and a bit of stomahesive to help it stick.then use the white sponge and cut it to put into the tunnel. I then cut a piece of the plastic tegaderm and put ot over this and cut a hole over the area where the white foam is. I then cut the black sponge and make a bridge to either his hip or thigh. This worked in the past, now, no longer. The wound is about 3x2.8cm and has a 2.3cm tunnel. Previosly was a bit wider and 6 cm tunnel.
So now I'm asking for any expert opinions. Is it time to remove the vac? If so, what now? The Doc I have is really not that experienced with wounds and takes my advise or those of us who work with wounds.
BTW, th wound is clean with nice healthy tissue, not really any maceration, and he has little or no pain. Again, there has been no drainage in the cannister for a couple of weeks.
I'd appreciate ANY suggestions. Would really love to give this man the opportunity to be up and out for longer periods of time and take advantage of PT!
Thanks!