Spinnaker sheet blocks

This is a question from a F17 Newbie. I must be threading the spinnaker sheet blocks the wrong way as are seizing under load and I cannot put any tension on the sheet. Are they adjustable?

Sounds like you have auto ratchet block.. do they seize in both directions? If the are in working order they souldn't. Run the sheet through each block and hold the sheet with each hand on both sides of the block, put a load on it an pull it back of forth through the block. It should ratchet in one direction and lock up in the other. The side that lock up should be ran to your spin clew.