@Mike London UK we can start to troubleshoot that here (and I’ll split the response into a new thread).
I asked around on the synth full and there is no option. One thing to consider is that you’d be adding a Full into the middle of some existing Incs, whereas those incs would not really be based on that interrupting Synth. Might not be a big issue, but definitely something to consider. Getting a Synth Full to run now would be the better option (and only option).
For your main issue, “why didn’t the synth full run?”, run a Job summary report and see what happened to the scheduled job. I am going to take an educated guess and say that there was an Incremental already running and the Synth full failed as a result. We have options for that if that was indeed the case.
I’ll split that reply off into a new thread for you so we can trace separate answers.