Installed Lutron Caseta, one bulb works, but two do not
There's a lot of garbage brands in the LED bulb game, and even if it's not a garbage bulb not all dimmer products are tested against all dimmable LEDs. One of these is likely your problem.

Fortunately, Lutron actually makes known compatibility information pretty easily accessible with a wizard that can be found here: http://www.lutron.com/en-US/Education-Training/Pages/LCE/DimmingCFLsandLEDs.aspx

If you are already using a bulb suggested by that wizard, then I'd start by trying to replace the dimmer with one that was already working inside to rule out a defect in that. If the dimmer is not defective and there's no compatibility problem, it must be either the wiring or the light fixtures themselves.

I've never installed one of those dimmers myself, but Lutron is a generally reliable brand. That's why I'm assuming it's incompatibility. But if you weren't using LEDs out there previously, it makes a wiring issue more likely - LEDs are more sensitive to wiring issues than other bulbs.