If I recall correctly, an oven flame failure devide keeps the flame low until the sensor has warmed up. Could be a failed sensor - or you've not let the thing enough time:-).
Update: Looks like I was correct. 2 minutes with Google (which YOU could have done for yourself!!!) gives me this: http://www.ebay.co.uk/gds/Gas-oven-wont-get-hot-/10000000020811842/g.html
