Foliage usually has a much bigger impact on UHF reception than on VHF, so that's kind of a stumper. Hazarding a guess, I'd say the 15-264's gain produced signals that may have been close to the digital cliff to begin with, and the marginal interference presented by leafy trees was enough to push reception over the edge.
That antenna is no match for any of its Winegard counterparts. More on that later.
Ray50's advice above is well worth pursuing first -- check the cabling and antenna for problems. Do you have lots of trees above or near the antenna? Tree debris like wet leaves or pine needles might be lying across the phasing lines, the cris-crossing metal wires above the boom and behind the corner reflector which carry VHF signals to the downlead. Anything wet that touches both lines could be shorting them, reducing reception. Lower the antenna and brush any debris off, and while it's down, make sure the phasing lines aren't touching each other or the boom anywhere. That would be a short, too. If the cable downlead is good and the antenna is clean, try Ray's pre-amp idea. The HDP-269 is a good suggestion.
If these troubleshooting steps don't do it, that's the time to consider a new antenna. Your signals are fairly weak, even at 30 feet up. I'd recommend getting the Winegard HD-7696P at a minimum; consider the Winegard HD-7698P for an extra margin of safety.
I ran a
TVFool report for the provided address. It's also attached below. (IIRC, you'll be able to do attachments beginning with your fifth post, so you're already there.)