Did you use the NHS booking page? If so it rejects you if you try to book before 182 (I think) days from your second jab. It could be that?
I tried to book for my dad, since we are getting no idea of timescale from our surgery even for my mum who is over 6 months and classed as extremely vulnerable, but it rejected despite being the exact required days. I'm about to try again.
Edit: now it says 190 days. It definitely didn't say that a couple of days ago.