Frank is a good coach. He can get the most out of his players, the problem is he can't consistently get enough good players on the squad to make a difference. I don't know if that's due to bad recruiting, or players just not wanting to play for him. However, when he does have decent recruiting classes, you can see what he does.
The four years recruiting prior to the Final Four (According to 247 Sports):
2013 - 5th rated in SEC (24th in nation) - this class had Thornwell, Notice and McKie
2014 - 8th in SEC
2015 - 7th in SEC
2016 - 11th in SEC (this starts the drop-off)
2017 Final Four!
The four years prior to last season:
2015 - 7th in SEC
2016 - 11th in SEC
2017 - 12th in SEC
2018 - 10th in SEC
And add in the latest class, and you have 4 straight bottom of conference recruiting classes and not a lot of hope for next season:
2019 - 12th in the SEC
Recruiting class rankings aren't definite, but when you're consistently at the top or bottom, the wins and losses seem to come accordingly.