I got a similar problem. The solution is the SOI (Sensor Of Interest), which needs to be on the weapon screen, a little rectangular dot that appears on the right upper corner of the weapon screen. You need to assign this to the joystick.
Now DCS is called a "study simulation", right? But sometimes it is more a "guess simulation", because the documentation is not really helpful. And the videos on youtube are way too long explaining a lot of stuff, but not putting enough emphasis on the important things. This is fortunately changing, because the community members are learning to create better documentations. But I would love to see a better documentation from the creators of the jets. For example better and shorter "quick guides", showing details of a joystick binding, keys, step one, step two, etc, with some typical problems you could encounter and so on. Most people I know who tried DCS was scared of the poor documentation, not because of the complexity of the simulation. Don't get me wrong. The documentation per se is ok (just ok), but it could be written in a way to save the reader time, to lead the newbie to the point he needs to know, not historical facts, not bla bla bla, but straight to the point where one is ready to use the staff. I write this after I collected the informations from different sources, some create PDFs made by users, not creators. I wish creators would make better guides than users.