I see this happen a lot. I'm assuming when you say the spokes are not straight, they're twisted? This happens when the spoke is not held when turning the nipple. They can make it true in the stand because twisting the spoke effectively adds tension , but as soon as the rider's weight goes on the wheel, that tension is lost and you're right back where you started.
A couple of things about Mavic wheels. They require special wrenches to true, and they have proprietary spokes. Proper tension is critical on low spoke count wheels, but a lot of shops don't even have a tension meter, much less a way to calibrate it to special spokes.
You need to find a wheel builder that works with specialty wheels, and is equipped to do the job properly. I'm finding your average LBS ain't up to it.