The Tott back piece connects to the base in two places. The main connection is at the very base and is comprised of amouldedhook that grabs on and often takes a bit of force to attach/remove.
The second connection which seems to becausingyou issues is not so much a lock as a guide. What this means it that while the parts should poptogether, it is notstructuralas the part further down with the hook system.
What we have found is that every now and then the seat fabric can move and impede this guide from joining. An engineer here who specialises in car seats andsafetystandards showed me how to make sure the holes were clear and then simply pop it into place. To illustrate her point she wiggled the fabric around so it wouldn't connect, then cleared the holes so it did, and then also stripped the base fabric off to visually show me how it pops in.
To ensure the guides will pop in you could strip the base fabric back and try, this shoulddefinitelywork. Then try again makingsurethe access holes are clear. If you have any doubts about you seat fitting in the car, head to your local Plunket/Safe2go centre and they can make sure that it issafelyinstalled..