STOVE NEWHOME GAS COOKER, BLOCKED OR DIRTY OVEN JETS, HELP!!!?

hi , my mother in law has a stove newhome 500 si dl gas cooker and the oven bit takes hours and hours to cook anything!! once before i sprayed some cleaning product down the jets at the back and it did get really hot for a few cooks then slowly went back to being slow! can i remove the jets to clean them or any tips on what i can do to clean them so that it stay getting really hot, everything else has been checked and the fact it did get hotter after dousing in cleaning product tells me its just where grease has blocked up the holes etc.....! help!