blackberry bold 9900 faulty says insert sim card even if its inserted !

blackberry bold 9900 faulty. even if the sim card is inserted it says insert sim card i dont know why.it came in a job lot its in fully working order apart from the fault mentioned the phone is not bl0cked i have checke it on checkmend the report of checkmend will be given to the buyer i am located in hounslow west / west london contact 07852822877 ali