Unable to mount weapons on vehicles??

Hi
I have a, LV Lupus 4x4 which is a Valkryie with a passenger weapon mount. Also a CDF Helicopter which has a side weapon mount and a Saehrimnir which has a weapon mount on the roof. I used to be able to mount weapons on these. I bought a MRL VA30 and Porcupine MAP-13 and I cant mount either of them. Any ideas why not?

you must be sitting in one of those vehicle slots to equip the weapon that fits it, not all vehicle weapons mount