USB stick not always recognised

My MBPR is a year old and has started being picky with USB sticks. Sometimes they'll mount and sometimes they won't. A restart is the only way to guarantee that they mount. I bought a new stick thinking it was a faulty one, but exactly the same with the new one.

Doesn't matter whether the sticks are formatted XFAT or Mac. I've reset the PRAM and the SMC which works for a while, same as just restarting does. El Cap 10.11.5

Any help or ideas appreciated...