Delivered means that the message has been pulled off the queue, but hasn't been fully processed yet. Usually, this is due to a problem somewhere in the processing logic or an unhandled error performed by the component in question causing the component to hang, die, or be stuck in a loop.

The <STORE> error means that the process has run out of allowed memory and failed out.

My guess would be that something is stuck in a loop that sets more and more variables until the <STORE> error is hit, failing out the process and leaving the message stuck in the delivered state. However, you may want to open up a case with the WRC for more detailed analysis, as that is primarily speculation based on the limited data available.