It is unlikley that the filter is blocked (not a common problem), but not to be ruled out. I would be looking to replace the oil pump as they are common for not working. You have to remove the clutch and drum and the oil pump is held on with a few screws. Unscrew and remove the pump assy. Once the pump is removed you will see the hole going into the oil tank, allowing the filter to be pulled and cleaned. I would recomend that you fit a new oil pump to cure the problem.
Neill T
July 2008