Before checking the following, we always advise that you have the log splitter on a flat surface. If needed it can be on a slight uphill but never pointing downhill.
The operating valve is blocked or stuck: Check that the end of the valve stem stays in contact with the face of the lever when the lever is depressed and released. If you find the valve does not pop back when the lever is released, then you will need to clean it. Before removing anything please ensure the ram is fully returned to prevent any damage to the ram. Then remove the nut which is holding the lever and then remove the lever and valve. Check the o-ring and gently clean the valve with a lint-free cloth. Replace the valve and the lever.
The operating sleeve is incorrectly aligned: Remove the valve as mentioned above. Loosen the grub screw in the angled hole below the valve sleeve then use a punch to tap back in the sleeve and tighten back up the grub screw.