I don't know how long you've had it out, but I find lobelia to be very short-lived. Try cutting it back hard, but don't be surprised if you lose it. Lobelia tends to do better in partial sun only, but even so, if you buy good-sized plants early in the season, they don't make it through the summer. At least not in my experience.

Janet's Garden

I get the 'Moon series' lobelia. I've had very good luck with them lasting the whole summer. We get weather in the 90's and they're still good. I have mine where they get about 4 hrs of morning sun. I mulch them and water deeply once a week. I have mine in a raised bed and they hang over the edge. I trim them every so often, but not all at once...just here and there. I've tried other lobelias and none have lasted as long as these have.

Kat