The last two items entered for bid were not executed despite yet both should have been successful. In the first instance, the item sold for more than $50 less. In the second there were no other bids and the item went unsold???

Hello Paul,
Thank you for your question about your bids. To see the error(s) we encountered when we placed your bids, on your "Completed" tab. There is also a "log" icon showing the full history.
It seems you had revoked the authorization. Most likely, you changed your eBay password. We do check for that situation 8 hours before the bid, so it must have been after that time.
To remedy, just visit our Preference and click the "Log in with eBay" button.
Please let me know if I can help further.

Thanks for your reply, but the explanation is not valid. Password was not changed in the time frame noted. Furthermore on checking preferences, I see that authorization does not expire until July, 2024.

eBay is the authority here. They rejected the bid because your token is not valid. They don't tell us why. I said "most likely," but we are not told the reason.
The expiration in Preferences is a guideline. We cannot constantly poll all authorizations, unfortunately.

If I'm paying you for a service, it would seem you have an obligation to provide the service purchased or address the issue when you fail to deliver. It is pretty lame to leave me in the dark and simply ignore the issue. Quite annoyed about the two items recently lost, as in each I was the high bidder.

This obviously means as much to you as it does us. Please cut and paste this letter to eBay.
"Hello. I used a sniping service to place a last-second bid on item ____________. They tell me the bid was rejected because the "Token was revoked." I have pressed them for diligence; but they insist that they are only relaying the message that the eBay API returned to them. Questions: 1. Can you confirm my bid attempt was placed? 2. Please tell me how the token was revoked?"
If you didn't change your password, eBay can also expire a token due to a security concern or unusual activity or something specific to your account. We check the token periodically up to 8 hours before the bid. There is also a "log" icon next to the snipe, which shows the full second-by-second account. That is all real and not fabricated.
I'd appreciate hearing what eBay had to say about it.