This is because you've applied your red base coat straight onto the black.
Because red is such a light colour, it doesn't go very well onto dark surfaces such as your black. There's a rule of thumb for painting colours over others: dark colours go smoothly over light colours, but light colours don't go well over dark colours.
So your whites, yellows, reds, pinks, etc. don't go well over your blacks, browns, blues, greens, purples, etc.
The colour of your primer can help, so if you are having serious issues with painting your reds or want to give your model a lighter finish, I'd recommend a light coloured primer such as white or red.
But, if you do want to stick to black which is completely viable and will give your model a more darker finish, you need to gradually transition your model from black to red.
I use a black primer for my blood angels so what I do is paint brown onto my primed models, leave to dry then hit it with a layer of brown + a little bit of red. Repeat this until you're satisfied with the brightness of your red basecoat.
Then you can highlight, shade, etc. It also sound like you're having a spot of trouble with the thickness of your paints. Remember to thin all of your layers down with some water.
EDIT:
I've just seen your reply:
Ragingkittycat wrote:Thx essia for your help!
So can I just respray them white as they are? All will that clog up the details to much?
Please. Do not do this. EVER. If you want to start with a new primer, strip your model down with some Simple Green or Dettol before you do. Your mini already looks thick and spraying another layer of primer will not make it look better.