We had few fire types introduced in two generations in a row (Gen III and Gen IV). We had some in Generation V, but they aren't still many if compared with other types introduced in the same generation (water, grass, bug, ground, rock and steel, for example). Furthermore, don't say that we also have few ghost or dragon types, they aren't like fire type. Fire is part of the grass/fire/water triangle, and the ammount of fire type Pokemons is much lower than grass and water.
I really don't know if GF has some trouble on creating fire type Pokemons, the fact is that we have few of them. Plus, Magmortar is the only fire type with a thee-stage evolutionary line that isn't a starter. This reflects how is hard to create a complete line of fire type Pokemons.
In Generation III, besides Torchic's line they introduced three fire types (Numel, Camerupt and Torkoal).
In Generation IV, besides Chimchar's line they introduced only Magmortar (if you don't consider Heatran, because it is a legendary, and consequently hard to catch).
Some things I have noted that Nintendo probably had done to fix the lack of fire types:
* To fix the rarity of fire types in Hoenn, they made Vulpix and Slugma available in the wild.
* To solve the "unavailability" of fire types in Sinnoh, Eevee is obtained as gift from Bebe, so players could evolve it into Flareon.
Well, post your opinion if you also think that GF has some "trouble" on creating fire types and why.