Oven & hob taking a long time to come on(on...

Question

When initially igniting either the grill or the oven, both will ignite but ''gas'' is only on very low.
Sometimes it takes up to 3 or 4 minutes before they ''fire'' up and I get full pressure.
I only use ONE at a time as per makers instructions,if it's the grill I always leave the oven door open,and if the oven the door remains closed.
Cooling fan is operational.
Can you please help? why are they on what I would term as ''low pressure'' ?
regards Mr M Cartwright
west midlands.