The timeshift buffer is quite fragile. It will be dumped in preference to making a timer recording and in many other situations, too. So it is not something that should be relied on. If possible, set timers and record the programmes you want to watch, even if you think you might watch it live. And if you find yourself watching something that suddenly becomes interesting, convert the timeshift buffer (if it exists!) into a proper recording by using the Recording item in the Main Menu and selecting the "Current Event" as the Start Time (some replacement EPG TAPs may also offer this feature).