I was a little too keen and eager previously and sent my sample chapters off too soon. Since been rejected, understandably. I totally underestimated the editing process. I have now redrafted the entire story and was wondering is it frowned upon to send it again to the same agents?
I've heard you should wait six months or more before re-submitting to an agent, and only to do so if your work has been substantially changed and/or edited from the original submission. I've also heard a well known agent say she doesn't remember the rejected manuscripts, so that might work in your favour! I must admit I did the same as you, sent my first book out too early when it wasn't ready - easy to do when you're excited about your work.
Good luck with the submissions :-)
You may believe your manuscript is ready, but I advise you to have your work checked over by a reputable literary service before you re-submit. It would be a pity to reduce your options. The majority of agents won't give a rejected novel a second look.
I'd have said to leave it a while between submissions if you're going to send to the same agents - and make sure it is different from eh last one right from the start. Otherwise they may be tempted just to junk it if they recognise it.
I'd also say, make sure it's finished before subbing, and as perfect as you can possibly get it.
Try other agents too; what have you got to lose? Check all websites for info first.