For all duch questions we need more info.
So for this one. What is the construction? Timber buards on joists?
What distance to the ground?
Have you a hatch or have you taken a board up? What do you see?
To tell you where my instinct is heading, I'd be thinking of spreading a polythene sheet over the ground to keep the moisture below it.
There will be a severe limit to what you can do and the sensible improvement.
You can test the dampness of the concrete floors or for £100 hire a boring machine so you see, snd show us, the construction and if there is a dpm.
Or assume there isn't, and apply a liquid dpm.