I've used Trusted House Sitters 8 times now and have had wonderful sitters. I never approach sitters but wait for replies to my advert from people who want to holiday where I live. I always get at least 10 replies (probably because of my location) so I am rejecting 9 others. I do message and have given advice on how they may improve their chances when applying for other sits.
Most sitters are not looking to sit locally as it is meant to be a holiday for them, so probably not worth contacting. I would imagine most sitters don't know when they are available months ahead so don't update their calendars but instead apply when they are available/interested.
I advertised my house site yesterday and have already a sitter lined up.
Sitters struggling to get a sit I would suggest taking a couple less desirable ones in order to get a few references and get the Enhanced Verification..