Check out the calculation control options and look for the following:
'Finish' Tab --> Finish Conditions --> Maximum Travels
You calculation may be stopping prematurely because of the limiting number of travels that are set. Try unchecking the "Maximum Travels" on/off checkbox and continue your calculation. I believe the default is 4? You can manually go in and change it but I generally leave it unchecked. Probably not the best idea if you have a batch of simulations queued-up since you'll get stuck on the non-convergent one but useful otherwise.
Thanks Dave! I have found that more recent calculations have been solving without issue (100%), indeed the previous ones (with progress less than 100%) seemed reasonable results, but I assume they cannot have been. I think your suggestion does make sense and I'll keep it in mind if this happens again - I've been using a less refined mesh lately, so this is probably why (given your suggestion) I'm no longer having issues.