Mobile Navi C-max

Hey, have the following problem, bought me a NAVMAN iCN330 yesterday, everything worked great in the car mounted. Now, however, the problem that the device partly only recognizes me next to the track and does not navigate me correctly. So I switched back to the Me..a Ma..t and took a higher value so this time a Falk N30 built again from the front etc. everything again wonderful, until on the reception he can receive no GPS signal, so again to the M.di. M…kt back and forth, the Service consultant couldn’t really believe it so out in front of the door and everything works wonderful, I think I can’t be so stupid and the device can’t operate properly, so the seller asked to go to the car and try it again and see it doesn’t work. He told me that it’s due to the beh. windshield and possibly to the warming glass, super money back but no Navi. My question is now someone can recommend me a Navi which in the C-m ax does not work for the two did it. Thank you in advance greeting Alex