How to fix my Brandy Best Gas raNGE?

I have a Brandy Best Gas Range. Mod.RU 196.50TFVEGTC..Cod.915343BZM...SERIE.N.0617000273.F. The problem i have is that when i press the knob in and ignite the oven which is fine but when i let go of the knob the flame will not stay on,i have had people look at it but say the cannot get the parts or they say they have never heard of Brandy Best..I really need help as my oven has been like this for 5 weeks now,i am desperate to get it fixed...Can you help me ?