Can connect Computers A to B, but not B to A. Not a HomeGroup Issue

This started as a HomeGroup issue, but seems to be more basic. Both 64-bit Win7 Home Premium computers were networking (wirelessly) fine using NetGear router, then I moved and switched to Uverse, then Xfinity routers. The issue is the same on both: In (Network and Sharing Center) => [Show full map], Laptop JOHN-PC sees the full map and after clicking on desktop MARY-PC can read all files. MARY-PC sees the full map but cannot connect to JOHN-PC--that is, the cursor does not change to a hand when hovering over it and neither right-click nor left-click is operable. I have Credentials set up in each machine to log into the account on the other: MARY-PC/Mary and JOHN-PC/John. But this is more basic than HomeGroup because even if I "Leave the HomeGroup" on MARY-PC and change its workgroup name to XWORKGROUP (different than on JOHN-PC), JOHN-PC can still read all files on MARY-PC. MARY-PC still sees the JOHN-PC icon but cannot access it. JOHN-PC/C: is shared.
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MARY-PC can ping to IPv4 address of JOHN-PC. and browses the web fine. I've checked out most solutions I could find on the web. IPv6 seems to be enabled in both computer and router. All HomeGroup settings seem to be right. I tried enabling NetBios over TCP/IP though I thought that was outmoded and flawed. I've checked the list of services that are posted as being required for HomeGrouping. MARY-PC has Atheros Dualband adaptor. I will be happy to provide any other hardware/software details. Any help would be appreciated.
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-Cab-
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