I used to have murder getting the bracket for my TrafficMaster YQ2 to stay put. The design of the mount basically puts the full weight of the unit into trying to separate the rear of the mount from the dash
Sellotape Outdoor Sticky Fixers worked pretty well, but can be difficult to come by. The normal (non-outdoor) ones are easier to find, but turned out to be next to useless.
Rear view mirror mounting pads were pretty good too, but they're expensive and by no means infallible.
And then I found the perfect mounting tape: "No More Nails On a Roll" Permanent adhesive tape. This stuff really sticks - just be sure to get the permanent (red backing) and not the removable one (a different colour - blue, I think.)
In all cases, you'll probably need to build up several layers of pads to match the contours of the dash - i.e. 2 or 3 layers thick on the outer edges of the mount, dropping down to one layer thick in the centre. You'll use a fair bit of tape, but the result will be a mount that stays put.