appendMediaEntry is sometimes NOT appending media entry to the rough cut

I went to the test me console.
Entered a ks, entryId, and mediaEntryId ... hit submit.
Inspected the dataContent field of the xml and nothing changed had changed ( no clip at the end)
Hit submit again.
The clip was now added at the end.

I don't understand -- why is this behavior occuring? I feel like this could be some server side caching issue or something ??