You don't indicate if it's going down or up. If it is going down it is because at the top it has a hole, with a filter or similar paper so that the air enters and the mercury does not escape if you turn it over. If it is rising and does not exceed 200 mmHg even if you increase pressure it is because the mercury in the tank is over, probably because it spilled and fell to the ground through a joint, from there you will see bubbles rise or the mercury in pieces, that is pushing the air and not the mercury from the tank, which goes just below the glass or graduated plastic column. These tensiometers can no longer be manufactured by the ROHS standard, which prohibits mercury, cadmium, lead, etc. If this were not the problem, comment and I will try to help you since I have repaired many.