Problem Wall Painting

I have a wall that won't paint properly.without streaks. I have painted it several times myself with different brands of paint and have even given in and paid someone to come and paint it for me. They sanded it all back and applied two coats of good paint but it still looks streaky. Someone told me that it was because the plaster is not level and therefore there is nothing I can do. The paint doesnt seem to go on well and even now you can see missing patches when close up. What I really don't understand is how it can be like this and be seemingly unfixable when it wasn't like this before. When we moved in it was also painted white and there were no visible streaks (not that it was bad enough to ever notice anyway.) If it's the plaster why was someone else able to paint it to a good standard earlier before we decided to redo it?