I grew Lavatera Trimestris Twins from seed this year and planted them early (April) out in full sun. They were gorgeous thru mid-June. Now, the leaves are getting shriveled and brown. It certainly isn't drying out, we've had a wet season here in the D.C. area. It is hot and humid now and I'm wondering if this plant is more of a cool season annual? I'm having the exact same problem with my Bells of Ireland. They started out beautiful and now are shriveling and browning up (although I notice the leaves here have tan spots on them first before they totally shrivel).
I have these plants in a huge bed up front with tons of other annuals that are all doing great with this weather we've been having (regular rain and sunshine).