1) Are you certain the Yanmar's parking brake has been released?
2) Do you have
four-wheel-drive engaged, or are you in two-wheel-drive mode?
3) Is your tractor struggling because the rear wheels are spinning, rather than holding traction? Are your rear tires filled with air or 3/4 filled with liquid?
~~~Or does the tractor feel like the engine is struggling for power?
4) Are your tires pressures correct? (On light tractors the tires can slip on the rims if tires pressures are too low. This is subtle.)
5) How heavy is the is the Bush Hog you have mounted? Does the Bush Hog tail wheel revolve freely? Is the Bush Hog rear wheel touching the ground most of the time when you are struggling up hill? Is the Bush Hog unitized to the tractor via connection of the
two LOWER LINKS and
one TOP LINK of the Three Point Hitch between the tractor and implement?
Your Yanmar is a sub-compact model. The it may not be able to draw a Bush Hog weighing more than a 48" width mower on steep hills.
6) What tires are on your Yanmar? What tires are on your Kubota?
7) What transmission does your Yanmar have? What transmission does your Kubota have?
YANMAR 424 SPECS:
TractorData.com Yanmar 424 tractor information
Note: Cat I/Limited Three Point Hitch
PHOTOS always help.