Backwards... weed brings out dxm. If you take enough dxm for it to do anything, you will NOT get high from your weed, you will trip off the dxm, only.
Smoking weed and taking dxm at the same time brings out the DXM, and covers up the weed. If you're not into crazy ass robotripping along with possible puking, it's not a good idea.
I love dxm, but it's not for everyone, and it will NOT get you high off of the weed.
If you can't get high anymore at all, you need to take a few days break. If it's just taking more to get you high, try smoking out of a bong or something that gets you higher than what a bowl or other piece normally would, if you don't usually use a bong. I hardly ever use a bong, so when I do I get wayyy more high than I normally would.
Hope I helped

good luck