Hi Diemaker, can I suggest a service, simply shut the main return pump down, the will drain the weir area to the sump, remove the head of the weir control valve, this is performed by removing the four hexagonal bolts, you will need to remove the cover caps fist, you can then check for any foreign material like algae build up, if the valve head is clean and clear simply replace the head, on start up, fully close this valve, this will push water over the overflow, you can then slowly open the valve again until you achieve the correct level (one notch down on the view window) I always look into the weir overflow looking for water cascading into the weir with the level 1/2 inch down.
If you continue to experience issues also ensure the flow pumps are clean and working well, the flow pumps can also effect the weir level if the the pumps are not running correctly.