I have just changed from a ground mounted Vertical antenna , to a MFJ-1796 Vertical antenna, due to not having big enough ground to bury as many verticals as recommended. So far what I have found is on FT8 the incoming signals are from the US as well as Western Europe … ie Spain , France , Italy etc., yet they never respond to my coming back to them. Since FT8 is low power usage , its not like the stations in Europe are using amps , so what gives ? One way propagation???
First: do you know about pskreporter? You're definitely getting heard, including one spot on 40m from New Zealand. Sometimes FT8 takes some patience, and it seems like no one wants to come back to you for long periods of time, but pskreporter can at least provide some reassurance that your equipment and the ionosphere are actually in working order.
That said, your antenna is dreadfully short. It's a multi-band "half-wave" vertical that's about 12 feet (4m) long. That's about 20% the length of a real half-wave for 40m, and 30% the length of a reasonable 4-band vertical (if you could imagine stacking two 4BTVs end-to-end to make a half wave and eliminate the need for radials, the contraption would be 43 feet tall). So it's not as efficient as it could be.
Assuming that want to stick with this antenna, you might want to try to maximize the efficiency as much as you can. Remove the 2m and 6m stubs if you don't use them. Tune meticulously for minimum SWR. Make sure that all connections are made firmly and free of oxidation. Use the best feedline you can get, and not a foot more of it than you need. Clear the space around the antenna of conductive objects as well as you can. Consider measuring the loading coils and seeing if you can wind better ones... you get the idea. Most of those changes are probably fractional-dB improvements, but enough of them can make a noticeable improvement, and sometimes small reductions in loss have a surprising impact on shortened antennas.
That said, I'd put my money on a ground-mounted vertical with "not really enough" radials still being a better performer than a 1796.