Yes, if you can’t maintain longer duration, that is a sign of an issue, IMHO.
The whole point of longer intervals is that it builds on itself and becomes more demanding the longer you hold that power.
Needing to pop one or two into a workout as a “bailout” to finish otherwise fully completed longer intervals is considered “OK”. But if you can’t even complete a handful of intervals over 6 minutes without breaks, something is not right.
Edit to add some potentially useful threads to review: