The Roman empire by the time of Diocletian was going through some difficult times. Just the sheer number of emperors suggests the rapid rate of change. In a word, there was a lack of stability. Part of the reason for this was the empire was too large.
In light of this, Diocletian acted in two very important ways.
First, he changed the structure of governance. He decided to divide the empire into two parts - the East and the West. This, in effect, made the empire smaller and so more manageable. He also had two sets of leaders. In other words, there were two senior officials called Augustus and two secondary officials called Caesar. This also helped (at least in theory) the problem of succession. The effect of this action had unintended consequences, as it created the Eastern and Western empires.
Second, in view of the debasement of coinage, Diocletian famously issued the edict of maximum prices. He did this to stem the problem of inflation.
In conclusion, Diocletian was an able administrator, but the problems of the empire were too big for one man.