The PCAT registration system is a bit silly. When you first register (and pay) they ask you to select the time frame you plan to take it in. Then when you get your code and go to the other site to book a location/time, they actually use that to restrict it so you only see the times available for that specific time frame. You won't see any dates for the other time frames even though there may be dates available for them.
A friend had a situation where they signed up (supposedly early) for the July timeframe tests and the closest location was booked already (small facility) for both dates. They couldn't even see the September dates. In the end they had to actually cancel their registration altogether and re-register selecting the September timeframe in order to see the September dates. That required paying for the registration again. They did of course get a refund for the original registration but the whole implementation is silly and nearly defies the entire concept of using online registration for convenience.
If you call, they can tell you if your location of choice has any open seats available for any test dates so you may be better off doing that instead of having to register, drop, re-register, drop, etc. Maybe if they get enough calls asking about seat availability someone will wake up and let people see seating availability across time periods before you put down money and get locked into a specific timeframe.