problems with extension tubes on my lenses?

[MHB]

i have a set of Zeikos extension tubes that fit well on my kit lens. i have two new lenses, Nikkor 55-300mm and Nikkor 50mm 1.8g. the tubes dont fit on those two lenses for some reason. maybe theyll go on if i force em but i dont wanna break anything. i was assuming it was because my two new ones have metal mounts instead of plastic but thats just a guess. anyway, is anyone in the same boat as me? is this normal?

 

[Edsport]

I have a canon extension tube and it screws onto the camera and then my lenses screw on to the tube. Any lens that will fit on my camera will fit on my tube. I'm not familiar with your tube but i'm guessing any lens you have that will fit on your camera should also fit on your tube...