Parkmobile is designed to provide a single Tax Receipt each time you park using the service (much the same as a retailer would issue you a separate receipt for each individual transaction, or a payment machine would give you a paper receipt each time you parked).
However, you can print an individual statement displaying all sessions for a particular from the 'Receipts' page of your account. This is not a Tax Receipt, merely a breakdown of all parking sessions for that particular month.