Is it in full sun? Plants can sunburn like animals. Glass blocks most UV radiation, so exposing plants taken from inside to full sun without letting them acclimate in partial shade for several days first can cause them to burn and lose foliage.

Or, it could be over-watering. Giving the plant extra water to "compensate" for warmer outdoor conditions and bright sunshine can rot the roots. I know. I've unfortunately done this several times--even when I should know better!

Thanks. This is their second summer, but it was their first winter. When I first got them I was told they like full sun. I'll try moving them to a shady spot for a few days and move them out to full sun gradually. I was thinking it could be the sun, but, like I said, I was told they like full sun! I actually got some good-looking flowers last year (when the plants were really small) so I'm hoping for TONS of flowers this year, the vines have really taken off. I love how hardy they are. One of the main vines is broken almost in half but there is still new growth on it!

They do indeed like full sun, but going from sheltered to exposed is a shock to the system. Give them a couple of days of partial sun/shade and they should adjust nicely. *IF* that's your problem. Could be something else, but that's my first guess.