Hello Massimo,
is this a permanent problem, or does the switch eventually get all the MAC addresses ? In the latter case, this might be a problem of convergence. You might want to try the ´uplinkfast´ feature (do not use it on the root switch though), which immediately sends dummy frames containing the MAC addresses that are in the CAM out the newly activated link.
To configure uplinkfast, use the following global command:
Switch(config)#spanning-tree uplinkfast
HTH,
GP
