Jack Li explains when you might not get a result back for query_plan in sys.dm_exec_cached_plans:
When you first execute a batch or stored procedure which may contain multiple statements. Not every statement will be compiled when you first use the procedure. If there are some code paths that result in the statement not being executed, SQL Server may choose not to compile that statement in a small set of scenarios. In other words, some statements will only be compiled upon first execution. So far, I have found the following two scenarios that a statement will not be compiled (deferred compilation) if code path result in that statement being skipped.
Statement involving temp table. Data could have been changed when the statement is actually executed. So it doesn’t make sense to compile right off the beginning.
Statement that has option (recompile) attached to it We need to recompile that statement every time anyway. If we don’t use, it why compile in advance?
Read the whole thing.