SD card reader within PC (fed by USB) sometimes fails to recognise that a card has been inserted

My Windows 7 PC has a front panel with various card readers (SD, CF, etc) in
a module that is connected to a USB header on the motherboard.

Usually the SD reader recognises that I have inserted a card and adds it to
the list of known drives in Windows Explorer. But sometimes it fails to do
so (it's as if I've not inserted a card), and the only remedy is to reboot
or logout/logon.

I'm not sure whether this also applies to the CF drive because I don't use
those cards.

It's not poor contacts because I've tried removing and reinserting the card,
wiggling it, pressing it up/down in the slot. No change. Once I've removed
the card and later reinsert it, Windows simply does not recognise that it's
there. And that is for a variety of cards. I've even tried swapping the card
reader module with one from another PC, and still the fault persists.

The SD-only reader in my laptop (which is also Windows 7) never experiences
this fault, so it's not a generic Windows 7 thing.