The Pioneers can take quite awhile to initialize, it's not abnormal. I don't fully recall which models do what, I seem to remember that some will initialize every power on cycle, later ones initialize after it detects the door was opened (?). If there is a problem with the players and the player is having a hard time reading the cd, it will take a very long time to initialize.
While my own opinion is that the Pioneer boxes were the best sounding at the time, the players in them just were not as robust as what others were using (Rowe/AMI, Rockola and even NSM).
If the player is timing out trying to TOC a number of cd's, (thus locking those cd's out) and the cd's are store bought (not CDR's) and clean, then there are a few possibilities. First would be a gummed up sled mech and it's taking too long to position the laser. Next would be gummed up spindle motor and it's taking too long to spin up the cd. Last would be a failing/weak laser.
The juke as a whole doesn't do very much in the way of diagnostics/error reporting, just some vague errors are available in service mode, so most of the troubleshooting is a guessing game. There are some diagnostic tests that can be run at the player level, but you need a RU-V101 remote control connected directly to the player under test to run the test commands. Those remotes are as rare as hens teeth.
If it's a failing laser or spindle motor, you will probably have a hard time getting parts, they just don't seem to exist any longer. Beware of used changers that pop up on ebay, more often than not they don't work.
Myself and others here have gone the route of the cdadapter MP3 hard drive setup for the Pioneers due to problems keeping the original cd players going. If you find that the problem is with one, or all, of your players I highly suggest the MP3 kit, it works great (disclaimer: the MP3 kit is FOR HOME USE ONLY).