I had a similar problem with similar symptoms. what happens is you start getting a mix of network ranges if you're serving DHCP IP's from both routers. then the switches (if you have multiple like I do) start "remembering" these and get confused on where to send the packets. if you put a sniffer on the network, you'll start to see packets colliding and going nowhere. you can configure things to be better behaved if the routers are flexible enough to do so, not all are.
best is to figure out which router you like best and put the other in Bridge mode (as I've done since I prefer the open source routing software I have on my old TP-Link router). What I'm finding I don't like about the Covr 2200 though is, once you put the "A" unit in bridge mode, it now
hides features in the GUI that are useful, trying to protect you. Not cool.
