You can make Neapolitan pizza in a higher domed oven, it's just not as easy and not really feasible in a commercial kitchen. For a home oven it can work, people do it all the time in full hemispherical Pompeii ovens with massively high domes.
My concerns point to the installation. Looking at both Valoriani's site and Mugnaini's(the likely distributor the OP bought from if they are in the US) it seems the proper insulation is an "option" and not a standard feature of the oven, and sometimes not even recommended.
This page shows the oven installed with 6" of sand under the floor in place of insulation:http://www.mugnaini.com/files/pdfs/2012_Mug_140_AOS.pdf
If you oven was installed per that diagram, that would be the issue. It looks like they consider the 140 a commercial oven. As a commercial oven where it is fired all day everyday, all that sand under the floor would haver a chance to get hot and stay hot. As a home oven where it is fired infrequently from a cold state all that mass will never get up to heat. The floor will constantly lose heat to the cold mass bellow.
If my assumption is correct your best bet will be to pull up the floor tiles, dig out the sand, and insulate, and then reinstall the floor tiles.