The way Microsoft originally designed the Windows clipboard, requires all applications that access the clipboard to cooperate. If one application doesn't play by the rules or crashes, this may affect other applications such as AceText.
Therefore, it is possible that the ClipHistory stops capturing text automatically, even though the automatic capture option is still enabled in the preferences. When that happens, right-click on the ClipHistory tab in AceText. Select "Reconnect to The Windows Clipboard" from the menu that pops up. AceText will then reinitialize its connection to the Windows clipboard, and the ClipHistory will work again.
Windows Vista introduced a new clipboard mechanism that works with each application in isolation. AceText automatically uses this mechanism instead of the old system if you're using Windows Vista or a more recent version of Windows. Since AceText doesn't lose its connection to the clipboard when another application fails, the Reconnect to The Windows Clipboard command is not visible if you're using Vista or later.