Why do sand martins sometimes reject last year's nest burrows?

The sand martins are back from Africa now and it seems to be a lottery whether they return to previously dug tunnels or whether they dig new ones

Each pair of sand martins digs a tunnel. The nest chamber, a human arm’s length or so underground, is safe from most enemies.

Ní féidir leis an gabhlán an dá thrá a phreastal (literally means 'the sand martin can't be on two beaches'... you can't do two things at once)

Having crossed Pharaoh’s hot dry ocean, ‘my’ sand martins are back from Africa. About 30 pairs nest each summer in a soft bank facing the sea, close to my home. The little migrants are weighing up their options... they deemed the site unsatisfactory last year and moved on. Will these temperamental breeders reject it again this time around? Fingers crossed!