Updated: May 17
I have been engineering Wlan and wifi in SE Asia for over 12 year and now can give to you some really important areas to consider making your access points and wifi more stable.
Roaming clients, some Brands work well, some are really terrible making your clients think your wifi is not stable. The worst method of roaming that many brands use is by actually disconnecting and them forcing a reconnection with the access point. This is usually called "fast roaming". Ugh, it is a cheap but terrible way. If your system has a choice of ways to roam, pick any other way and this problem may go away. Some brands of Wlan have as many as four or five ways to roaming, giving you the power to play and pick the best.
Roaming, if your access points are too far distant from the next access point, your client or phone will not be able to roam and actually will disconnect. Please remember, a phone may only have a 15 meter ability to stay connected to the access point, while a laptop can have a 50 meter range. So, arrange your access points within those ranges so there can be an effective "hand off" to the next access point. If they are further away, loss of roaming and more disconnects and more complaints.
In your configuration of the Wlan, you have turned on "air time fairness" this will disconnect all older phones and laptops that run of G series or older wifi, they will not be able to effectively roam.