It was possible to capture or seal higher-grade Spirits which were of a higher value than what Ash was capturing.
It just required a stronger seal and container to be able to effectively do so.
If a Spirit was of a certain grade and size then containing it would be far more difficult to do so.
It would require a lot of effort to put them inside of a container in the first place while the seal and container would have to be suitable enough to be able to contain it.
If it was not big enough then it would not fit inside and if the seal was not strong enough then it would not be possible to hold it for too long before it could break out.
Most would have to weaken a Spirit and capture it when it was weakened to seal it away.
Once they reached a certain grade, it was far more effective and efficient to just seal the area they were in despite the requirements to be able to do so.
