DO THIS AND YOUR GAS COOKER / OVEN WILL NOT BREAKDOWN

Figure 1 above is wrong way of doing things, as you can see any fat, oil, crumbs,debris which fall from food will fall on top of flame and therefore over time the burner will get blocked.
Figure 2 is right way, once again anything falling from food will not fall onto flame.
Do not place food right to the back of oven.

Get a good torch and a vacume cleaner and clean burner at the bottom back of oven. A thorough clean generally clears blockages and you will get a good oven flame again.