If you see "incomplete" for the ARP Entry, this is most likely the router is sending a ARP Request to the 3750 and is not getting a ARP Reply back.
If you have already cleared the ARP and still does not work, I would suggest that you do a "Deb arp" on both the 7609 and 3750 and see if you are seeing ARP Request and Reply packets. Based on the outcome of your testing, you need to focus on where the issue is. Maybe, what you are running into is a MAC Address limitation on the Chassis itself or the SP Network.
Also, it is recommended that you run the debugs after business hours.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...