You should actually be able to drill down into these issues via the Mist UI and find ways to solve this.
A few pointers:
Time to connect: primary issue is association and secondly authorization/dhcp. I suspect you have some clients which are basically out of reach further indicated by failed dhcp bindings - that is combined with some clients which fails authentication due to wrong PSK or similar.
Successful connects: You have one or more clients which keeps trying to connect with the wrong PSK or fails 802.1x.
For both SLEs you can click on association and authorization and figure out which SSIDs, and APs have issues and then analyze further from there.
If more input is needed please provide screenshots of the different panes after clicking on association/authorization. Example from a test installation:
