Yup - congrats from me too.
I had the same problem a few times previously, only involving electricity.

The reason is that you have some doubt, deep down. Often this happens in situations when you're put on the spot (by a DC for example).
It'll take a little practice (not that much) before you'll be naturally confident, but there are some tricks to help you right now:

Shortcuts: Instead of expecting the impossible (fire from your fingers), justify it in some way. For example, perhaps put on some expensive gloves that have flamethrowers on the fingertips. This rationalises it for your subconscious. If you find that the gloves malfunction, then avoid mech/machine shortcuts.
Other examples of shortcuts: a levitation pill, a jetpack in a box.

IMPORTANT: Once you've gotten the shortcut to help you, remind yourself that it's all in your head and THROW IT AWAY. This prevents you from becoming dependent on it.

Expectations: The reason you're having problems is because you have doubts, and kind of expect it not to work for some reason. Take some time to think about why that is, and convince yourself why it's rubbish.
You can combat those doubts in your dream, but it's a bit of a tricky thing. The natural thing to do is to focus on the problem and try to make it work. That usually doesn't work, because you're fueling the reason for the doubt.
Rather, you need to practice *not* thinking about it too much. Just expect it to work. Visualise it working before you even do it, and be confident that it will just work.

Don't think too much. Just do it.

At first these two tricks sound like opposites (one is to rationalise it, and one is to not rationalise too much)
The difference is that the shortcut trick places your confidence in something else (temporarily) while the 'just do it' trick relies on yourself.