The main reason tags or tag perspective groups are not appearing within reporting is due to the perspective group total hitting it's max.
Every perspective has a default maximum of 200 groups that resources can be allocated to. You can view the current total group count on the right navigation above the perspective groups allocated:
If you have a perspective that categorizes by a tag or other grouping that has over 200 unique key value pairs, the perspective will only be able to represent the first 200 and the remaining values and corresponding assets will not be allocated. This means that categorizing reports by a maxed out perspective will be missing data and will not be accurate. For instance, if you have a "Region" tag that happens to have 350 values associated to it, 150 region values will not be pulled in to the perspective and remain in "Assets Not Allocated".
There are two potential solutions to address this issue:
1. Optimize your perspective. This includes removing empty groups, consolidating duplicates, and overall ensuring that the groups you have allocated are correct.
2. Request an increase to the group maximum. These requests can be made to either your dedicated technical account manager or via a support ticket. Be advised that group limit increases may not be possible in every situation and may be denied if there is a risk to performance.
For more information on perspective maintenance, see: Manage Perspective Groups