If it's just been a couple of lessons I wouldn't worry about it. If it keeps happening then it's definitely something that needs working on and may be valid for postponing the test. However, I can't see why you can't book a test date and aim towards it and move it back if necessary. I tend to find once we have a date booked for learners, their driving improves significantly as there's now a target to aim for. You should also be able to add extra lessons in on the run up to polish up anything that needs a bit of extra work.
Definitely discuss with your instructor and between you I'm sure you'll come up with a plan.