Oriol

Start with the needle, because an adjustable one is the only part of this that sets loop length directly. A longer setting makes bigger loops, and plenty of people never touch it after unboxing. Shorten it, test on scrap, and find out whether that was the whole problem before changing anything about your technique.

The other half of your description, wildly different lengths rather than uniformly long, is a technique thing and it has two usual causes.

Check the fabric tension too. Punch needle wants drum tight, and monk's cloth that has relaxed in the hoop lets loops slip and shift, which reads as uneven length.

And look at the yarn feed. It should flow into the needle freely, so pull a generous slack length off the ball and make sure it is not catching. Any tug on the yarn and the needle drags loops back and forth as you work.