Full motor setup - Really wierd problem on 13e , 1 transponder random wont tune 0% ,

ok so my motorised system (tuner b on Duo) works great, i get perfect arc and can pick all sats up fine but for some reason all of a sudden, i cannot scan 11662 on 13e , one minute its 80% and tunes eg Hustler HD fine from the channel list , the next time its gone and Tune failed. I have no problems getting any other channels or frequencies other than this? Cant be the LNB (black ultra) surly??? totally lost!