Bummer. Here's a list of things I'd try:
Verify your offset. Channels 23-30 should already be set to +5 Mhz.
If you're inside, get outside and away from nearby buildings. I can receive just fine sitting at my desk, but I can't transmit to nearby repeaters unless I'm sitting in a very specific spot (thus the mobile mount on the porch).
Make sure your radio actually works. My dead TD-H3s act like they're transmitting and I can pick them up from other rooms in the house but not across the neighborhood, much less hit a repeater 25 miles away. Either send someone out with a second radio to make sure it works or get a cheap SWR meter and make sure the HT is sending power to the antenna.
If everything checks out and you still can't get to the repeater, you can try a 771 GMRS antenna. (Edit: could also try to build a rat tail counterpoise if you're feeling feisty, but if you don't already have a wire stripper/crimping tool, you'd probably spend as much on wire, terminals, and tools as you would on a 771)