Pages indexing well then dropping

[ugandangeorge]

Hey

I have a fairly large site that is performing great (for pages that have been there a long while) but i don't seem to be able to get any new pages ranking well for their specified target phrase.

I don't think its sandboxing as the pages are immediately indexing well, number 1 for most phrases, then dropping (as if it were sandbox) but never coming up again. This is not effecting pages that have been uploaded for some time nor it is happening to all pages. Most are just indexing but not coming up in serps. But... this is if i search for the target keyword, if i search for something else, even making a small alteration to the search they are coming up and normally at number 1.

Can anyone help?

Thanks