Try removing all the prefilter cartridges and run the water filter for a few seconds.
If it is still puts out very little water then there is a clog somewhere in the line. (Or you have a ball valve set to the wrong setting somewhere).
If it puts out the expected water volume without the prefilters, then you can try to isolate the issue.
Start installing the prefilters one by one and run the water again. Start with the sediment prefilter and work your way to the DI cartridge.