The below video discusses and demonstrates the most common reasons why the Pressure will not be able to achieve target within the Inhalation Tower.  The four categories addressed are as follows:

 1)  Pressure is near 0, Flows are all correct:  there is a leak in the tower, a component on the tower, or tubing near the tower.  Verify that all tower ports are plugged (either in the open or closed position), the little ports for air purging on the photometers are capped, tubing is pushed in tightly everywhere and free of leaks, and the components on top of the tower (or on the side table, if using an elevation platform) are screwed in tightly.  

 2) The pressure is higher (in the positive direction), Inflow is normal, Outflow is lower than expected:  there is an obstruction in an Outflow pathway.  This could be due to compound clogging the tubing itself or a pinched tubing (including the back of a Pull Supplemental Flow Unit), but more than likely the TSI HEPA filter has become saturated and needs to be replaced.  To test this, temporarily remove the HEPA filter and connect the Outflow tubing directly to the Outfow device (Tower Controller or Supplemental Flow Unit).  If the outflow and pressure can achieve the target, you know it is the HEPA filter that needs to be replaced.

 3) The pressure is higher (in the positive direction), Flows are all correct:  there is a leak in the outflow pathway, or the inflow is too high.  The inflow being too high can only happen if there is an electronic issue with the pumps/valves/sensors, but a leak in the outflow pathway can happen for any number of reasons.  Most commonly this will be a tubing that is not quite pushed in all the way.

 4) The pressure is lower (in the negative direction), Flows are all correct:  there is a leak in the inflow pathway, or the outflow is too high.  The outflow being too high can only happen if there is an electronic issue with the pumps/valves/sensors, but a leak in the inflow pathway can happen for any number of reasons.  Most commonly this will be a tubing that is not quite pushed in all the way, but could also be a component between the inflow tubing and the top of the tower that is not screwed in all the way. 

Another possibility for exceptionally high or low pressure within the Tower: If when setting a tower pressure of  minus 0.5 cm H2O for example the pressure within the tower rapidly rises to a high level or when setting a positive pressure of 0.5 cm H2O the tower pressure rapidly goes negative, the problem could be that the incorrect transducer has been used. Systems with head out plethysmography will have been supplied with flow transducers for the head out chambers, These are not interchangeable with the transducer intended for the tower pressure. The correct transducer for the tower pressure will have the part number 010150-001.  

Finally, if the pressure is slightly in the opposite polarity of what you have set as a target, this may be because the pressure transducer is flipped around.  On older towers where the transducer plugged into a manifold and was oriented vertically, make sure that the cable side is pointing upward.  On newer towers (the stackable design) where the transducer is plugged into the tower directly and oriented horizontally, be sure to align the + and - symbols on the back of the transducer with the matching symbols etched on the tower.  

 As a general tip, if you cannot find the issue quickly it can be wise to start removing components from the tower to make it simpler.  If you are using a Cascade Impactor, Gravimetric Inline Filter, Gas Analyzer, Diffusion Dryer, and two Photometers, start removing one component at a time until the pressure stabilizes at the target.  By removing one component at a time you should be able to ultimately deduce where the leak is and fix it much more easily.