All too often we see startup's that seem interesting but may be missing certain metrics and therefore we decline to invest.
In cases like this I always say to keep in touch.
Investors are always influenced by momentum from other investors so if you manage to convince them you should not be shy about sharing it.
Regardless, if I ask you to keep in touch, I mean it. It's not a brush off. Keep me in the loop on fund raising, milestones met, product/market fit and so on. I may be unconvinced on your initial proposition but early stage startups change almost daily.
A good policy is to have a weekly/monthly bulletin that is sent out to an investor list.
Too often, entrepreneurs take rejection personally and should look beyond that emotion. Investors are swayed by a mutliple of factors outside of your pitch, some of which you would have no control over as they are internal processes.