My understanding is that it may depend on the interface you are using. There might be timing issues and ODBII was not really as standardized as you might think when it first came out. I THOUGHT however that around 2005 all vehicles were supposed to be standardized on a common CAN bus protocol with some ID's and such preset but much is left to be OEM specific.
You should be able to get things like speed, temp, o2 sensors, trouble codes, MAF, MAP, and some other standard sensors but beyond that depends on the vehicle you are reading. There are a bunch of different ODBII "standards" that came out before CAN was supposed to standardize everything.
You can always put another ODBII scanner on your vehicle and track whats going on from a laptop or something to debug your software and see what is going on.