DD is 9 and bright. Good academic ability and a good level of self motivation. She's not exceptional, as a friend of ours is (he's studying for 11+ and is ridiculously motivated to work for it). My dd won't do stuff at home unless she wants to and always does homework last minute.
I was a SAHM from her being 4.5 yrs. Prior to that I'd worked from her being 14 weeks old. I taught through play during the early years and still do so, when I can, though it gets much harder as they get older.
I do think she had some benefit for me being at home during the formative years...it gave her a headstart. However, if you've read the Hare and the Tortoise you'd be INSANE to think that a headstart means you're onto a winner! We have had a good start and I'm grateful for that, but how things go from here is up to my girl, her attitude and (to some degree) her teachers. It's very little to do with me.
The top 10 kids (from academic results only) are from single parent families, traditional family units, have working parents and in fact, I suspect I'm the only SAHM there was. That kind of blows holes in the theory.
I personally think it doesn't boil down to what you do in the day for work, it comes down to the small pockets of time when you can teach informally or formally.
And, of course, as many have pointed out...kids sometimes bloom a little later, so it's never as cut and dried as it may appear.
Stop beating yourself up...you're a great mom or you wouldn't be worried about this and if you gave your kids the choice, they probably wouldn't want their lives to be any different...