It's occured for me first under win 7 & now under win 8, on 2 different PC's using both G-Blade & Orbs. I've looked at power supplies, Com Port settings, cabling..you name it. IMO its some sort of 'initilization issue' as either the miner/s will kick off and start accepting shares or they wont (until next time bfgminer is restarted & the device/s are reinitialized), though I have seen cases where a miner will accept a couple of shares then nothing.
If you are using MultiMiner and have it set to restart "suspect" miners it should pick up on miners that have a low effective hashrate.
Also note that, depending on the pool difficulty and luck, it may take a bit for things to play out.