Actually, you used the right process in adding DE via the skimmer. What you may have done wrong was to not have the multiport valve on filter. Perhaps you had it on re-circulate in which case the DE by-passed the filter and went right back into the pool. Not sure if this would happen in the backwash position or not. If you did have the multiport valve in the filter position, you may have a hole(s) in your grids. If that is the case you can sew them up with some fishing line.