Hole in forecast assignement

Hello all,
I have just reconciled the complete planning of all projects and I have a strange gap on the assignment of some resources. It doesn't happen for all resources.
The attached picture shows this lag for resource 6

E.g. the activity 764 has no predecessor (only successors), is planned "as soon as possible", but the guy is not assign as soon as the previous activity (on another project) is done

What kind of trials can I do to understand where is the bad constraint?

Thank you for any help