No cheap Chiltern late evening Advances from Birmingham in February ?

[General Zod]

I've been looking at Chiltern Advances from Birmingham Moor St to High Wycombe / London for travel in February. Usually there are a plethora of cheap tickets for the last three Chiltern services out of Moor St but throughout Feb and the tail end of Jan one can only secure Advances for travel out of Moor St prior to 1800 ? There aren't and evening engineering works scheduled so could anyone explain the sudden change in pricing policy / lack of competitively priced late evening tickets ?