Miller Big 40 diesel will not start unless I use starting fluid. Then once it starts I can start all day long until I let it sit over night. Then I have to use starting fluid again. How do I fix this so I don't have to use starting fluid

I am not familiar with the engine.

Indirect injection diesels need a cold start device in all but the most mild of weather conditions - mostly this is accomplished with the use of in-cylinder heater plugs but these might or might not be supplemented with an excess fuel device or an intake manifold warmer.

Direct injection diesels lose much less heat during compression and so an engine in good condition will usually start without aids in temperatures above freezing though most engines will incorporate an excess fuel device, manifold heater or in-cylinder heater plugs to cope with low temperatures.

The first thing a diesel needs for good starting is a good cranking speed and excellent compressions followed by good injector spray patterns and that is usually enough to ensure reliable starting of a direct injection type.
An indirect injection type is even more needful of good compressions and a good cranking speed and also requires good injector spray patterns but will also need any cold starting aid to be working properly.

I suggest you begin by learning about your engine.