A lot people are going to be mad at me.
Don't lose any sleep over it. No one will be mad at you, I promise.
I think W9BU might be on the right track as far as the time goes. I don't use the N3JFP program either, but if it converts your local time to UTC, you might want to check that it is taking Daylight Savings into consideration when converting from local to UTC. With LoTW, your time and the other station's time must be within 30 minutes of each other in order to constitute a match but if the times are 1 hour apart because you are not converting the time correctly, you won't get a confirmation.
One of the things you can do is to look at one of the contacts from your logbook made after 7/26/2015. Open the LoTW website and click on the "Find Call" tab. Put the other station's callsign into the search box and click "Look for callsign". After you do this, it will display the last time that station uploaded a log to LoTW, if at all. Of course it will not tell you what is his log (this is part of the double-blind system) but it will tell you if he uploaded a log after the two of you made contact.
If he uploaded a log after the two of you made contact, you might take a shot at looking him up on QRZ and see if he shows his email address. If he does give his email, then you might want to sent him a friendly note informing him of the details (time, date, band, mode) of the contact you had with him and ask if you are in his log. If he is a nice guy (which I am sure he is), he will respond with the details from his log and either they will match yours or they won't. If they do not match, then you will be able to see what differences there are between your logs.
If there are differences between your logs, try this with a few other contacts from your logbook and see if there are any trends among them that might help give you a clue as to what might be the cause of the non-confirmations.
Clear as mud, eh?