coverage failed, means that a number of clients connect to an AP with a signal strength lower than a set limit during a percentage of the session. This does not necessarily there is a real problem, or that the client can not work
It indicates fro some devices the connection may be intermittent, so not guaranteed for sustained data-flow.
cause of this may vary, but needs further investigation.
a client-session report may give information about what-clients and what access-points are involved.
or monitoring specific clients may give some information.
You need to match this with the physical location where the client is and if this is a location where wireless signal should be available you can expect that the signal is reduced if a phone is put in the drawer of a desk!
This client does not or can not switch over to another access point in, but keep in mind it is the client driver that decides to "roam" to another access-point. So not in all cases it can be solved bij adding or redistributing access points, sometimes a client driver deeds to be upgraded or configured.