Issues getting into Maintenance Mode on the PFD is often times directed to the GPS/Nav/Com radios.
Turn off the radio's. Once the PFD understands it is not in flight mode (active flight plan), you can use the line select keys (LSK) to enter Maintenance Mode.
Issue - I am holding down the left LSK 1 and 3, getting the count down timer, it gets to zero (0), I let go, and still not getting into Maintenance Mode.