Possible thermostat installation and replacement issue?

I replaced the thermostat on my car but I noticed in your diagrams that you show , the thermostat sits back further into the housing than my thermostat because in my housing there is a plastic piece. I'll call it an adapter that sits in the housing also and it only goes so far in the housing causing the thermostat to stick out making it hard when putting it back to together or did I do something wrong because I had to loosen the water pump to get it back together? Thank you. Andreas