DE Pool Filter Tank Not Draining?

I have an in-ground gunite pool. I have a DE filter. Despite having a relatively clean pool, I have to clean the filter almost every two months or so. Every time I clean, I release the air on the tank and when I remove the tank lid, the water is automatically drained.

This time is was half full and had sediment in the bottom. I tried to unlock and raise the backwash valve just to let water run out. The valve would not budge. Although it may be unrelated, my far skimmer is also not sucking water. If it is clogged, I couldn't imagine with what. The skimmer basket wouldn't allow an obstruction to pass.

The water stays clear. I need to know - is it OK to put the filter back in and backwash before putting new DE in so that some of the sediment will drain out along with the let over water? If not, then what is the remedy? Also, I never refilled my tank with water manually with a hose after I put the filter in. I always just turn the pump on. I have seen some instructions that say to manually fill the tank before putting the top back on. What do you think the problem might be?

By Cleve from Houston, TX