I have three equipments in the same LAN. One 6500 switch, one 2800 router and one very old 1600 router. The thing is 2800 and 6500 can establish EIGRP neighborhood. Neither 6500 nor 2800 can establish EIGRP neighborhood with 1600 router. 6500 and 2800 added 1600 on their Neighbor tables, but 1600 didn't.
I run "debug eigrp packets hello". All of them send Hello messages, but for some reason 1600 does not see Hello packets from other equipments. 1600 is connected via serial interface with other router and they could establish neighborhood. I wonder if it is some issue with 1600 Ethernet interface.