That is a lot of stretch, actually.
Are you sure you have the same number of links on the new chain?
Are you sure you have the right chain?
You might clean and lube and adjust the derailleur to see if you can improve it.
You might also ride it for a little bit to see if the chain/cog marry better.
But it sounds like you need a new cassette. Many people change chain/cassette together as a matter of habit. I would have hoped that 900 miles would not have required this, but with that kind of stretch, I think that is what you are looking at.
jim
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