Auto transmission sometimes does not go to Manual

[spiezzy]

hi to all I have just noticed a few weeks ago that in my Navara STX550 2011 7 speed Auto that when I select Drive and then select Manual mode it sometimes just stays in D and won't select Ds or full manual until I turn the car off at the ignition the start her up again and all is good again it only does it every now and then but it can be frustrating hauling up a hill towing and I get the problem any ideas what could cause this any help would be greatly appreciated
cheers Pete