Help--Thin Brick Woes!!!

Hello all,
I have just purchased $5000 in thin brick for a 3ft high facade all the way around my building, just under some old vinyl siding (we cut three feet of the siding off at the bottom).
We stapled on black tar paper, then--don't laugh--screwed on cement board. FWIW, USG claims that their product is fit for use in applications including "exteriors" and "brick veneer."
Every kind of mortar we have tried on the cement board has failed to keep the brick on solidly. The brick sticks into place just fine, dries, and then falls off with the slightest vibration (e.g, a door closing) even a day later. We tried to add a scratch coat on top of the cement board with the same results.
We have tried Portland Cement grades I and N mixed with fine sand to a consistency similar to stiff sour cream.
What are we doing wrong? If you can, please tell me how to do this without removing the cement board. (PLEASE, PLEASE...)
Thanks to all the home repair gods that deign to reply!
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I hope you installed the cement board with the rough side out. The smooth side should work, but the rough side preferred.
Make sure you use sharp washed masonry sand. Do not use play sand.
1 part Portland cement (not the masonry cement you have been using) 1 part lime 3-5 parts sharp clean sand. Mix to the consistency of cool whip. It should be able to stand up on its own, but should be fairly easy to squish.
Dampen (not flood) all surfaces before you start.
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Dan G

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Dan G,
Thanks for the suggestions. We do have the cement board rough side out. We've seen threee grades of Portland cement: I, N, and S. Which do you suggest?
As for the sand, Home Depot has "Commercial Grade" sand in Fine, Regular, and Coarse. We've been using Fine grade.
The addition of lime and the wetting of the surface is new to me. Does this create a quicker/tighter bond?
Thanks again!
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Portland comes as type 1 , type 2, type 3, etc.
N and S are masonry mixes.
Lime adds a "stickiness" to the portland. Contact a concrete company or brick yard in your area about good sand.
Get enough materials to to make a trial batch or two.
I had not thought until someone else posted about your substrate. If you can push on the wall and flex it, you will not win.
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what is the substrate under the cement board? If it is just black board or energy brace, then you might as well screw the cement board to the studs.
Even with 1/2" OSB, I'm not sure the wall is rigid enough to prevent vibration from or wall 'give' to loosen the brick.
I have had a similar problem the first time I used cement board over plywood for tile flooring.
My guess is that you would need at least 3/4" plywood screwed every 6 inches before putting cement board over top of it. Does USG or GP have any installation requirements? They must.

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I beleive you should be using thinset mortor like you would use for tile.
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