LyX ⇒ Cross-References aren't working properly

[kleifest]

I'm writing a rather lengthy document in Lyx (in the AMS article class) and have been trying to cross-reference labeled definitions, propositions, etc. I want the reference to show up in the text with a capitalized name and number, such as "...according to Definition 6..."

I added the following to the preamble:

\newref{claim}{name=Claim~}
\newref{defn}{name=Definition~}
\newref{prop}{name=Proposition~}
\newref{thm}{name=Theorem~}
\newref{lem}{name=Lemma~}

and use formatted cross-references when I insert a reference.

It does exactly what I want for some of the items, but sometimes the references will show up with question marks where the number should be, such as "...according to Proposition ??..."

Does anyone know why this is happening or how I can fix it?