If it is still too difficult, it is possible that when you were not using PowerLung model you may also have decreased your other activities and simply are not in the same physical condition you were in. Remember, like your other muscles, your respiratory muscles can lose strength when you do not train them regularly. You may want to consider purchasing a Resistance Cells Set for a model with a lower level of resistance and use those until you build up your strength. You can keep the Resistance Cell Set from your existing model and use them when you are stronger and ready to use them again.
Typically there is nothing wrong with your PowerLung model. The first thing you should do is wash your PowerLung model following the instructions in the PowerLung User Instructions.