If the valve is not opening or seems to jam during a training session it is mostly likely that you are failing to overcome the training load. This could be because the training load set is to high or you are fatiguing during the training session. Reduce the load until you can perform 30 breaths.
Make sure the valve is not sticky. If the valve is blocked then the valve may not open. Refer to your manual, section 11 Care and Maintenance.
Ensure that the valve head is attached correctly to the POWERbreathe K-Series unit. Failing to do this will mean that the gears will fail to mesh and therefore the valve may not open or close. See Figure 1