These factors can all cause inconsistencies to the prints if not monitored:

The humidity - it may be useful to have a humidity reader next to the Mac®hine, so the customer can monitor the consistency of this

The temperature of the heat press - this is usually stated on the front panel of the heaters, however it may vary across the width of the bed if faulty. Use a probe to measure the temperature across the width of the bed, and not just rely on the overall temperature reading on the front panel

The fabric coating on on the belt feeding the fabric through - this can wear down and flatten over time. When this happens, the inks on the sublimation paper are not able to evaporate properly causing areas of the print to look lighter than others

The speed of the Mac®hine - this will depend on the type of paper going through the Mac®hine. For example if you are using the production paper (usually 60 gram) the temperature will be higher and the dwell time (i.e. the time it takes from beginning to end transfer process) will be faster. If it is paper for digital (120 gram) this is thicker, so has to go through slower  and at a lower temperature so that it does not burn the fabric. Sometimes due to the demands of production, the temperature and dwelling time may not be amended by the operators when the paper is changed, so this is something to bear in mind as it can impact this can have on the quality of the colours at the end.