Often I see the following issue, updates are installed during a Task Sequence, then the Task Sequence doesn’t continue after a reboot. In the smsts.log are errors like these:
TS::Utility::GetTSMDataPath( sDataDir ), HRESULT=80070002
Failed to set log directory. Some execution history may be lost.
The system cannot find the file specified. (Error: 80070002; Source: Windows)
Executing task sequence
Task Sequence environment not found
Then you should check, if some updates require a second reboot and are listed in KB2894518. Because this second restart is not controlled by the task sequence, no execution state is saved before the restart. And when the task sequence resumes after the restart, no state is available to continue successfully.