I guess you're using the radio with the internal VHF aerial.
The signal level might seem to be more than adequate, but as the EM81 magic eye works fine of AM and not on FM, the signal level from the internal FM aerial may be insufficient to operate the eye, which requires several volts negative on the grid to appreciably change the display. I'd suggest that you check the negative volts on the grid (Pin 1) of the EM81 with a station tuned in on MW strong enough to operate the eye, then tune in a VHF station at seemingly good strength and check the negative Volts on the grid.
I think you'll find that whether on or off tune, the voltage won't change much on FM.
I say this because some years ago I restored a Pye Fenman 1 which worked fine on AM and FM, apart from the eye not opening on FM. I checked all the voltages and resistors associated with the eye, and all seem to be as it should be. As the Russian equivalent of the (EM80) eye were cheap at the time, I bought a replacement, but to no avail. I made a little tester on which I could vary the negative volts of the grid, which proved that the old and new eyes worked fine.
Then it dawned on me that I should try an external FM aerial. Ironically, I had a ribbon FM aerial in the workshop 'hiding in plain sight'. I plugged it in, and the eye then worked fine on FM. I probably spent more time faffing around until the penny dropped than I did to restore the radio.
Likewise, I restored a little Unitra 'Figaro Special' (AM only) which worked fine on its internal Ferrite Rod aerial with stations at good strength, but the DM70 eye didn't alter much. I checked the voltage on the grid, both on and off tune. Off tune it was 0.45V negative. On tune, it was 0.75V negative. With a short external aerial plugged in 2 Metres of wire), it was quite a different story: 4.5V negative off tune, and 9V negative on tune, when the DM70 'exclamation mark' eye closed completely. The volume didn't change greatly, with or without the external aerial - the AGC did what it's supposed to do and controlled the gain.
Might be worth a try with a simple FM aerial - just a length of twin speaker wire to mimic an FM ribbon antenna would do. They're cheap enough to buy if you don't have one:
https://www.ebay.co.uk/itm/251486153...IAAOSwKClidqZr
Hope that might help.