How to improve efficiency of the diamine synthesis from fluorescein?
What I think is happening is that you are not getting enough mixing between your reagents, the solid remaning is unreacted potassium carbonate. I think you can reduce the amount of potassium carbonate you are using - you only need 1 eq as it is dibasic, and use finely powdered if you have access to it. Run the reaction more dilute with a more powerful stirrer (have you an overhead stirrer?) so that material does not stick to the side and follow it by TLC. If this does not improve the outcome I would consider using a strong organic base such as DIPEA or DBU in place of the potassium carbonate so that everything is in solution.