Updated: May 17, 2020
Actually there are many, many reasons for clients losing their connection, I will try to list a few, starting at the top. Here is a short list:
Too many clients on access point- this will cause queuing of the clients, and slowing of signing on. Cause is usually, very low memory on access point due to cost or utilization of existing memory.
Client is too distant from access point- You may see this more with phones, not so much with laptops. Installation engineering problems.
Cheap, low quality Brands of access points, not much to fix on this one.
High RF interference, you will experience this during high density configurations, such as meeting rooms, conference halls, stadiums, bus stations, etc. Normally caused by too many users in small areas.
Client issues, such as old client, low memory, no password, too distant.
Roaming problems, even when standing still, due to poor roaming software engineering on access point.
Roaming being controlled by distant controller, look for roaming controlled only on access point.