[Stvrob]

You can retry the rootstock if the first scion fails, But don't wait too long to decide! If you graft above the cotyledon you would have the option of leaving them on so the rootstock would be livelier.
Are you using expensive rootstock?
None of this really goes according to plan. You can graft way up hi On the rootstock. I had the same problem. If it works you can bury it deeper when you pot them up.

Here is a better boy rootstock with a cherokee purple scion on the left and a new Brandywine scion on the right. The rootstock was way way ahead of the scions and I was getting discouraged because eveyday brought them further apart in growth. The cherokee purple was cut nearly at soil level and the betterboy was cut way up in the foliage.

The other pic is one of the few "successful" grafts from before I got the silicone clips. It wasnt lined up very well and has never grown out of the kink in the graft. I hope it does OK in the garden.