Try changing some of the slic3r settings under 'Print Settings' --> 'Support material' --> 'Options for support material and raft'
You'll need to be in 'expert' mode to see these settings.
My guess is you might need to decrease the pattern spacing or the contact Z distance. Extruding a little hotter might also help.
Also how many raft layers are you doing? Slic3r typically wants to do 3 interface layers, so I'd do 5 layers for the raft.
If you're still having trouble posting a picture of the failure might help me give you some more specific advice.