High DP is due to a reduction ...

High DP is due to a reduction in the open area for the feed flow. As the other commenters have suggested this could be due to an inadequate cleaning procedure. It could also be due to entrance of non-dissolvable materials (bacteria, silt, sand) into the elements. How are your elements arranged? Are you seeing the same type of DP across all of the vessels in the array, or are you seeing higher DP in either the first or last vessel? In the last vessel it is more likely to be precipitation of some compound from the feed water as the concentration increases. In the first vessel, and especially in the first element, it is likely to be something coming into the element with the feed. Could there have been construction debris left in the cleaning tank or piping?