It sounds like air has leaked into the compression chamber, which means there is no means of pressurising the system.
Please follow the steps below in order to rectify this issue:
1. You will need to release any residual pressure by loosening the hose nut on top of the compression chamber to allow the air to escape.
2. Next, undo the nut completely and remove the hose and dip tube, de-couple the metal arm link from the top of the chamber, unscrew the tank cap, and remove the compression chamber from the tank.
3. Turn the compression chamber upside down and pour its contents back into the tank. Make sure it is empty before re-assembling.
4. Slide the lance, hose and dip tube through the tank cap and refit these into the compression chamber, making sure the nut is fully tightened down by hand.
5. Re-fit the compression chamber into the piston.