I cut it back and trimmed it. Gave it a shot of iron?
Why Are the Leaves on My Geranium Turning Yellow?
Certified GKH Gardening Expert
I would check for water. Make sure that it is not too wet with no drainage and also check to see if water is penetrating the root ball. Sometimes soil can start to repel water if allowed to be dry too long.
As a precaution, also treat the plant with a fungicide as this could also cause yellowing leaves.