Could be a kinked tube going to the gauge, or it could be an actual boost leak. A turbo will "spool" (spin) as long as it has exhaust hitting the turbine wheel. However, it won't make boost if there is no resistance (i.e. a leak) in the system. If you are still good on power, not having smoke issues, and your EGT's look fine, then I would say there is a problem with the hose/tube running to the gauge. Running a boost leak test would confirm or deny this as well. Pressurize the intake and look/listen for any leaks.