Why can't I defend my settlement?

This happens. I've had an instance where there were definitely no enemies and in that instance you're just going to have to deal with it.

Essentially, explore the entire region around the settlement and see if you can find more enemies, if you can't, sit around for a bit and see what happens. If you have a siren then use it, see if the call to arms allows one of your settlers to see the enemies that are supposed to be attacking.

Settlement attacks are random rather than pre-determined which is why reloading the save didn't allow you to relive the attack.

Additionally, settler count and settlement statistics via the Pip Boy interface are not always correct, as discussed here in this question.

Pip boy shows wrong information all the time and shows some crazy numbers that are very often incorrect. If you suspect that your sttlers could die, you should always fastravell and check it yourself. Te only way to get 99% proper information about settlers is when you are in the location in workshop mode. Also if you got to the place and attack does not happen, you can try some tips that sometimes can trigger it. Run around the place. Sometimes enemies can spawn but they don`t attack untill you trigger them by coming enough close. When you are in the location open pipboy and fastravel to this location again (yes, you can fastravel to location you are already in).