Your problem most likely is that the tubing you are using, while it is made for high temp, is being crushed by the weight of the grain bed.
I have that type of hi-temp silicone tubing and it should only be used outside of the tun or any other vessel.
What you want is the opaque and more rigid silicone tubing.
Follow this link to see what I am talking about....
Change your tubing and you should get great results, like I did today.