Log splitters varies in strength and a weaker machine cannot handle harder woods.
The splitter should run smoothly through the wood, if it runs unevenly or stops, that would mean that it cannot handle the type of wood or the thickness you are trying to run through the machine.
If the machine cannot handle the piece of wood, turn off the machine and remove the piece.
Firewood from deciduous trees are easiest to split when damp.
Firewood from conifers are easiest to split when dry.