Okay, So you could buy vehicles for free? That doesn't sound right. Just make sure that the factory's relationship is friendly. If that doesn't work, Capture another factory and try again.

Do you have enough resources in your factory to actually make the vehicles you want to build?
I had the same problem and became frustrated with not being able to build my vehicles EVEN though the factory was classed as a friendly one. One day, by a sheer stroke of luck, I walked up to a damaged/disabled vehicle which was near to my truck and pressed the 'Y' key. The damaged vehicle disappeared and I was told that the resources of the vehicle were sent to the resource pool i.e. to the factory. After doing this several times, I managed to build up enough resources to eventually buy my own vehicle. Try this method. It should work.

@jamiegrover1973
I will give it a try. But is this really necessary if I have already enabled all assets?

@Lolwutted
No joke, I was surprised too. And yes all captured targets are friendly and activated.
By the way the factory was the one left of the map. I think it was north of an powerplant between the hills.

@Sacksahne
I think @SaOK tried his best to balance the game/mod out. Otherwise, when you captured a factory, you could simply build anything you wanted and trample all over CSAT in a matter of hours, using tanks, etc that could be manufactured in an endless supply. Atleast this way, you have to 'farm' for the resources. Afterall, metal doesn't come from thin air so @SaOK has made you gather the necessary assets/resources in order to build your vehicles and to slowly produce the vehicles for your army. No factory has ever had an endless supply of metal/ores. I personally like @SaOK's novel way of doing things.