Some Johne Deere front loaders have two pumps, an OC pump in the belly and a CC on the front it. It sounds like the one of the pumps is not putting out enough or you have contaminates in the priority valve.
Change the filter, remove the priority diverter valve on the left side of the tractor located between your left foot and rear axle make sure everything is clean before during and after.
Disassemble valve you will find a passage with a piston and spring remove and clean it. Remove the spring and put the piston back in bore and see if it moves freely back and forth. Run some 800 grit crocus cloth around piston and in the bore lightly. Clean up and blow out with air. Reassemble spring and piston the way you found them, install on tractor and try.
NOTE do not run tractor for long if the loader is not working this will ruin your .
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