Can't get a plastic dowel to get grip on a plaster wall... desperated

First I'd like to state I'm not a native speaker so I'm struggling to find the technical names, excuse me for the errors :/

I've been hanging some new furniture but I'm having real trouble with one of the clamps, the dowel keeps falling after some time or just rolls when tightening the screw into it, I think the problem might be in the plaster, that it breaks due to the pressure the screw makes. I've already hanged another, heavier furniture with the same dowels in the same wall and I had no problem but now I'm afraid it could fall down suddenly at some time. I've tried also using chemical anchor but it also failed. I don't know what else to do to keep it from falling... any pro ideas out there?

Thanks in advance.

EDIT: Picture Imgur

Furniture: http://ift.tt/1Vz7k0i

Submitted January 31, 2016 at 06:32PM by Imnpsnm http://ift.tt/1nW7rZv