Ralph - I had that problem with the smaller Wallenstein
chipper - BX42S. Combination of pine sap and green needles would plug the output chute. Until I found the complete solution - a bone dry limb every so often usually did the trick. This smaller Wally was mounted on my Ford 1700.
The complete solution - identify, fell, drag to pile - chip the FOLLOWING year. Allowing my small( 1" to 6" on the butt) pines to dry out for a year solved the problem with the smaller Wallenstein.
Now I have a bigger
chipper - Wally BX62S - on my updated and bigger tractor. I can immediately chip the pines now. Considerably larger discharge chute, more HP, much higher air flow thru the
chipper.