workgroup computers can’t see each other

We just added a computer to a pre-existing and perfectly functioning workgroup. It can file share and ping with all but one other computer, but it and that one computer are not recognizing each other in any way. (Yes, the 2nd computer is still sharing just fine with all the other workgroup computers.) If I view network computers and try to access either from the other, I get, “not accessible…might not have permission…the network path was not found.” I’ve been through all the Microsoft support online suggestions. Any ideas?