That's not completely out of the ordinary.

Most likely you will need to charge it at a higher than normal rate, and monitor it, to get it to start accepting a charge if it's below 10V. A Tender may not be able to supply enough Amps, but you could leave it on for a day and see what it does.

It will take a significant amount of time to charge if it's completely dead - possibly something like 12 hours at 2 Amps (depending on the size of the battery). Always keep track of how hot the battery is getting while charging, and back the Amps down if it gets very hot to the touch.