Keys Not Sending?

Anyone else having people say that after they won a giveaway that they didn't get a key after you use the "Send Key" button?

I had one in my last batch of giveaways that contacted me through Steam saying they didn't get their key after I clicked the "Send Key" button, it was on a 2 copy giveaway so I thought maybe the way I copy and pasted the multiple keys from my Notepad of keys threw off the system or something even though on my end everything looked great and as if it sent the keys. So I wound up taking the time to e-mail the remaining 3 winners in that batch that didn't mark their keys as received...

But than today I had someone message me on Steam about http://www.steamgifts.com/giveaway/SUarf/truffle-saga saying they didn't get their key either (yes I checked, it was the same username/avatar combo) and it was a single copy giveaway so the only thing I can think of is I sent 10 keys in under a minute for all of my ended giveaways, maybe that caused an issue with some of the keys going to the winners? As of now I got 7 open key sends which I've unmarked as Sent and re-clicked Send Key (most of these users look like they may not of been on since I've sent the keys though) in hopes this would "re-send" keys that may not of been sent. But I'm curious if anyone else is having issues? People whom have done alot of giveaways at once and have had this happened is it because you tried to send multiple keys in a short span of time? I also wouldn't put it pass someone trying to whine about a key to get "another" key so that was another thought, but I really don't think it was the case in my two PM instances.

Really hope to figure out what's causing my issue with sent keys because I have around 70 keys filling up my extras text file at the moment that I want to get rid of, but between having to wait for sent keys to get marked received to be able to start the next batch of giveaways (doing them in batches of 10+ giveaways at once is just easier for me), than after ~3-4 days having to go through and e-mail everyone who hasn't marked their keys as received because people are telling me they never got keys so I assume others may not have gotten keys also is a headache for me. It makes me think about going back to just posting extra keys on my Facebook page again, but between knowing the keys are actually getting used/enjoyed and having won some games on here myself I'd rather keep making giveaways here until they become too much of a hassle with getting the people their keys. After winning http://www.steamgifts.com/giveaway/wbzB5/painkiller-hell-damnation but never getting a key I try to ensure people ALWAYS get their keys from my giveaways, hence the going out of my way e-mailing after a few days or after getting PM'd on Steam even though this goes against the main reasons I registered here in the first place (a quick, easy, HASSLE FREE way to get extra keys to people whom actually will use them, posting on Facebook is easier but I'm sure there's likely a key or two that went unused and my friends are either cheap, poor, or don't game on a PC so it's a one way giving fest when I post there heheh)...