This, unfortunately, can happen. The first and most important thing to do, although it may be late in your specific case, is to make sure you choose a chair that fits on the seat of your car, since due to the design of the seat, some chairs may not adjust well. If necessary, and you are in time, you can return it and exchange it for another one, since it does not serve its purpose (if it is not used and in its original packaging, you should not have any problems).
Very important: slight looseness may be normal, but if the seat moves a lot you should not use it, because its main function, that of restraining the child in the event of sudden braking or an accident, may be compromised and it will not offer the same level of safety.