We have had swallows nesting in our car port for the last 5 years but this year there is no sighn of them, although I did see a couple flying around and also perching near to the carport earlier on .
It's difficult to give an answer, it may be the original nesting pair(s) haven't returned, natural life expiry, predation or they've found somewhere else.
Hopefully I've given this a boost so someone more knowledgeable may be able to give a more definitive answer.