Could have been just a bad day that happened because reasons (i.e. you didn't feel good, you were particularly stressed, whatever), you should take a look at your previous months and see how you did.
If on your 5+ weeks you did 10 reps every month then this time happened to hit 4, it might just be a bad day, happens.
Most likely, you need to tweak your TM. Pick a TM that allows you to do 5 good reps (85%TM, most recommended usually) or 3 good reps (90%TM).
You might not need to test it, if you keep your training logs (and you should) chances are you could have hit 3-5 reps in some "+" weeks PR, probably the 1+ week, so that's your number.
Whatever you do, I'd finish the cycle you're doing anyway so you can have the full picture with all numbers, PR sets and such.