The way you grab can vary depending on percent, but typically wobbling out of a dash grab requires your opponent to not mash, as it takes a while for Nana to get back into position to start the wobble. However, there is some talk going around the board about having Nana uptilt out of a dash grab while Popo jabs to lock the opponent in grab stun, which seems like it should actually work quite well (or at least better than just waiting for her to slide back). Other than that, it's best to have Nana be as close to Popo as possible when initiating the wobble to minimize the amount of time the opponent has to break out of your grab.
There are a couple of things that could be going wrong if you are struggling to wobble properly. I personally felt that initializing the wobble was the hardest part, but what I did to finally start landing wobbles was to immediately start tapping to the beat of the wobble, and f-tilt with Nana on her first possible hit. This ensures that the opponent has no window to break out of your grab. If you're dropping your wobble at higher percents, it means your rhythm is off. I'd suggest slowing down the wobble a bit if that's the case, as 200 BPM isn't the only timing required to wobble. I believe it starts in the ballpark of ~175-180 BPM, if I remember right.