...
The TM matches from the old project are set as not approved and you do not allow using not approved translation memory in your current project.
The new project was created before the TM entries were created, and you did not open XTM Workbench in the Edit Mode yet, so a match update was not triggered.
You are using a TM penalty profile that penalizes matches from the old project.
The old project was created under a different customer that the new project does not have access to.
The language combination is not exactly the same. It perhaps uses different variants.
The old project is using an MT workflow with no Linguist input. By default, XTM does not save raw MT in the TM base. For such matches to be saved in the TM, we either require the Linguist to, at the very least, accept the TM match, or introduce an Approve TM auto-step to the workflow so that these matches are saved automatically (for more information, read the following article: How to Set Up a Project to Start and Finish Automatically).
The TM tab gets stuck upon selecting a customer
...