first tank with a sump?...you have a section with filter media...
remove all the media from it...
fill up the water until it reaches about 3/4 of the height on the return section:
while it's on add your media and this way you'll find out what blocking your sump flow
and what media you might have to replace with a coarser one
here's a picture of my tank with an internal sump built by me...
lower water on the last partition (return) just means you need to add more water...this actually gives you a visual indicator on how much water you lose to evaporation
and you don't get water marks on the main section of the tank as it's always kept at the same height..only the water in the sump fluctuates on height
if the water is flowing over the partitions themselves then your filter media is clogged
water changes should be done by lowering your water 1inch above the height of the pump and filling it back up
or by turning off the pump and doing a bigger water change like 20%