- The respective SignalTap file must be enabled for the design. This is done automaticly in most cases, but you should check it.
- You may have downloaded a different file, e.g. a same name *.sof from a different directory.
- The configuration may have been reset by the on-chip configuration controller, or by an external hardware (e.g. a microprocessor or external configuration controller). On-chip configuration controller can be halted by a tools option. Disabling external controllers may be a problem with designs, that didn't consider debugging requirements.