"Forget" all but your primary network
From the device-level settings, navigate to the wifi menu and confirm the device is connected to the correct network.
For any other connected networks, tap "Forget" or "Forget this network". This prevents the device from hopping between signals.
Check network frequency
Try experimenting with your network frequency. Some folks seem to believe running dual band will work better on the 2.4GHz instead. Others swear by 5GHz.
Set a dedicated network and channel
Larger offices have had better luck when the devices are on a semi-private network, instead of sharing with hundreds of employees and thousands of other laptops, tablets, phones, etc.
Upgrade your OS
Some older versions of Android and iOS have known issues with wifi. Upgrading your OS may help.