That's not the problem. The centre caps on most of the Wu series big cubes sometimes come off very easily. It's especially bad on the Wuji, but it also affects the Wuque, Wushuang and Wuhua (v1; don't know about v2). This seems to be caused by inaccurate turning: relying on corner cutting too much applies some pressure to the side of the centre caps, which leads to them being dislodged.Problem 3: Pops
Solution: Tighten the face
However, if they're coming off literally all of the time, it's very likely that you have broken centre pieces. Especially so if you've been trying to force a misaligned core to fit.