I can identify with your frustration! Both myself and my TA dealt with Flight Ease and Lufthansa with the exact same issue - not assigning our return flights’ seats for our July 2-16 cruise. After 2 months, we finally were able to secure seat assignments this week.
This is what we did: log into Lufthansa with your Lufthansa confirmation number (not Flight Ease). On the page which opens, you will see “seat details” on the far right. I’m doing this from memory but if tickets have been issued, you should see a ticket number (it’s either on this page or the one where you select seats). Follow this to select seats. Of course, it won’t let you and gives you a message with a telephone number to call in a box in the upper right hand corner of the page. We spent 2 days calling this number every 30 min until it was answered. We then spent over 30 min on hold. Once a representative answered, we were able to secure our seats.
Please forgive the lengthy explanation if this is what you have done. If you have not called this number, I hope this will work for you.