electric fired gas fireplace
The "tick-tick" is likely an electric ignitor (a unit with a standing pilot light would not normally tick like that). Sounds like the ignitor is not consistently producing a spark capable of igniting the gas. The unit would be equipped with a safety feature that would close the gas supply valve if no ignition was detected during the "ignite phase".

Check the condition and position of the ignitor assembly. If none of this sounds familiar to you, get a gas supply company representative to check it for you (they often will inspect, and sometimes repair, for free; they want you to use gas because that's how they make $$).