You may be able to reverse the lines between the loader lift arms and the bucket curl to see if the issue follows the lines. This may help isolate the problem.
I had the exact same problem with my JD 3520, and it turned out to be a stuck control lever that connects to the spool valve. The stuck lever would not move far enough to the left or right, so the spool valve was not allowing fluid to flow to the bucket lines. The control lever worked just fine in the forward and back directions, so the loader lift arms worked as expected.
I initially thought is was an issue with the spool valve. Thankfully, the service shop was able to un-feeze the control lever and lube the connection to the spool valve. Much less expensive than a new spool valve.
Thanks a lot. Correct, the blue quick coupler wasn't seating properly. In order to be able to set it wright I had to release the pressure on the line coupler. Then, I was able to set it wright. You people are very helpful.