Hi,
I'm new to lock picking and can comfortably pick padlocks on my shed

I then tried picking the lock on my back door (upvc). But I have encountered something I cannot find information on. I can only assume it is a security measure. I can "zip" the lock and turn the barrel with the tension wrench, but rather than unlocking, the barrel simply rotates and there seems to be some sort of security spring loaded thing preventing the lock from being picked.
Does anybody know what I mean? And if so could you please provide your knowledge of what I have encountered?
Thanks,
DJ