I might be wrong, but I'd be willing to bet that the OP's problem is not really knives at all. He didn't say what model he has, so it's hard to tell, but what he's saying sounds familiar to me. What I have found a little frustrating with my Woodland Mills WC-68, particularly on large hardwoods (3"-5" diameter), is that they tend to refuse to feed, meaning they haven't even got close to the knives yet. The grips on the feed drum tend to cut the bark off the very end, but cannot dig into the hardwood like it can pine, so won't grip it and pull it in. The cleats are a straight edge (not very sharp) and simply won't bite on hard wood. What I usually wind up doing is feeding a smaller piece first to get the drum raised high enough that it will climb up on the larger material. This takes some loading speed skills and being very ready because the WC-68 will DESTROY a 2" limb of any kind of wood in a blink. I usually slow the feed drum down as slow as possible yet still enough flow to turn it. This accomplishes two things: I get the second piece in before the first is gone and it lowers the feed rate so I don't stall the tractor. IIRC, the Woodmaxx WM8H has dual infeed rollers. I know I looked at at least one model that had dual rollers. That will have a very clear advantage over a single roller machine, but I couldn't justify the cost differential since I'm never going to be using this thing for an income source.
That said, I am considering some mods to my infeed drum to improve the grip. I want to cut notches in the edges of the 'blades' of the drum, off-setting them so that no two notches line up, and sharpen the resulting teeth to a point so that they'll grip the hardwoods a bit better. Not stupid sharp that it becomes a wear/maintenance item, but to the point it will grip more like a peavy or tongs when wood gets there, and at a minimum, hog off some material on the end and finally grip enough to climb up on the doomed limb so that I don't have to use a two-stick infeed technique.
Tightening the infeed springs in my case would only aggravate the issue I have. If the first blade on the drum doesn't grip the leading edge of the material, it just starts hopping and making a lot of noise. This also normally makes the stop-bar shut off the infeed hydraulics. In addition to using the 'Two Stick" technique described above, I've also started trying to cut stuff that typically has infeed issues on a diagonal. Make sure to cut it so that the taper is turned up with the limb curl down (cut angle with the curl), or the limb will roll and slap ya pretty hard when the drum grabs it.
There is actually a third technique on longer straighter brush that can be stripped of limbs if the size warrants it. I keep a pair of lopping shears nearby for stuff that I know is going to give me troubles, so sometimes, if I have a really long piece, I clip all the limbs off and feed the little end first. That obviously doesn't work if you leave the small brush attached, though, and CAN lead to overloading the tractor if the infeed rate is set too high as the material gets larger in diameter.
It still beats the heck out of a 6.5 HP 3" chipper/shreader. It's a lot faster, for sure, and a lot less larger wood waste that I have to leave in a pile to feed termites before I can get it reduced to mulch/compost.
