To summarize from a Cuban who is mid aged and studied political science:
Democrats messed up in several ways, especially when Castro died. Apparently some of our leaders like Karen Bass and Bernie Sanders made comments about Cuban healthcare and literacy advancing under Castro. That was an affront to the Cubans.
Calling Trump a dictator but then publicly normalizing some of his policies rang hollow to many Cubans and Venezuelans. These are people whose life and world view have been shaped by dictatorship. Castro for Cuba and Chavez and Maduro for Venezuela. If you fled either and then you hear someone praise some aspect of their regime its going to hurt you. Moreover, the FL Democratic Party had a clumsy announcement about the embargo hurting Cubans without actually pointing out that Castro is what was hurting Cubans. Again, it rang hollow. There are other examples.
I can speak on this first hand. Ive been married to a 2nd gen Cuban for 30 years. My spousess parents and family fled Castro during the revolution. For them the Revolution changed everything. Fortunately, my wife and her parents hate trump. They are the exception, however. There are some in my married family who have fallen prey to the right wing talking point that Dems are communists. So, theres a little part of me who can see where the Democrats tripped up. Remember, Hillary carried Miami-Dade County by 30 points. We can retake the county with work and the correct message. Normalizing ANYTHING about Castro is a no go. It would be like saying at least Hitler built the autobahn. Thats horrible and would never be done.