Hi Sofia,
When BitLocker stalls near completion on a large drive, it usually means the encryption filter driver is waiting on pending disk operations. The first thing to do is check the drive status with manage-bde -status X: to confirm whether it’s paused. If it is, try manage-bde -resume X: after a full reboot, since a restart often clears the busy state. If it still won’t move, run chkdsk X: /f to rule out file system errors, and make sure your storage controller drivers and firmware are fully up to date. If the loop persists, suspend protection with manage-bde -protectors -disable X: and then resume encryption again. If none of these steps work, the only reliable option is to back up the data, decrypt the drive completely, and start a fresh encryption cycle.
Hope it helps.
Harry.