Could someone help me with load balancing, current stats:

I have a limited understanding, but from what I can make out, we have a significant number of clients being denied association and load balancing to different AP's but then the candidate count suggests a significant number of clients that failed to load balance, presumably because there wasnt an AP available in range that wasn't busy?
Uptime is 27 days, client count can reach around 220 at busy times, 63 AP's in the building 1142's, 5508 controller. I am wondering if increasing the window size would offer a more robust solution, or will this just degrade user experience further?
I am having a number of issues with failed client association now, and devices just not being able to operate wirelessly at all
Client count reaching 25 on some AP's