Just guessing here (we use another brand of GPSr)....
Possibly the "Send to GPS" function will not interact with the GPX file already loaded, each being a single and separate download.
I understand that with Garmin units, deletion is done either as ALL or NONE regarding the GPX file. Adding something to that existing GPX file (to me) runs counter to the "all or none" concept.
Solution? Set up an a weekly auto-run PQ for your home area, or multiples if you reside in a cache-dense area.
It's not a cure for adding a single cache, but that cache (new data) would be included.
We do have older Garmins for back-up but with those units, deleting/adding singles is easy.
I'm beginning to come to that conclusion ("send to GPS" will not update an older pocket query) which does make sense. I'll do a new PQ.
Thanks Gitchee Gummee!