CTC makerbot printing problems gap between vertical walls

[hunggaur]

hi folks new here and have just got my first 3d printer the printer was printing fine with PLA and i have now switched over to ABS and am now having problems with printing when it prints it does not stick the all the layers of the solid walls together its almost as if it is printing 2 verticals wall rather than one solid one as there is a gap between them its almost as if its not putting enough ABS down or the layers are to far aparti am using makerbot desktop slicer software using 4mm nozzle printing at 250 degrees the photo below should be one solid 2.5mm thick wall all round any advice welcomed as i am stuckmany many thanks Jon