Overflow weir making me irritated

Over the 5ish months ive been running a sump about once a month i have to shut down the return pump and let the water in my weir lower. about once a month the water level in my weir gets too high (water line inside weir rises to meet with the teeth of the weir instead of being lower allowing the water to fall into the weir) and surface skimming ceases and i get a film on top.



whats causing the water level to slowly raise to meet the teeth of the weir?



a few notes that might be important:

-i have the weir positioned a little lower then people normally would, the waterline stays at about 1 1/4 to 1 1/2in below the rim, so the waterline is lower then the trim 1/4th to 1/2in



-there are no air bubbles in the U-tube nor is there buildup



-there is an airline in the u-tube, it pretty much just stays there incase i ever have to restart the siphon, its also fully submerged



-overflow box is split in 2, one side has a drain, the other side always has water in it for automatic restart of flow incase of return pump on/off situation





would this be caused by the weir being a little lower, so the gravity effect is reduced going from weir to overflow box?



im pretty annoyed by this but searching has come up with nothing.