Proc Mixed with warning "unable to make hessian positive definite"

[SAS_solution]

ods output LSMeans=LSM;

 PROC MIXED data=adpro3  noclprint;

                CLASS trtn monthn usubjid;

                MODEL aval = base trtn monthn trtn*monthn /ddfm=kr s;

                repeated monthn / subject = usubjid type = un;

                lsmeans trtn*monthn / slice= monthn pdiff cl;

RUN;

But in the log the following warning information appeared:

unable to make hessian positive definite.

What's wrong with my coding? What is repeated measure analysis?

Please help. Thank you!

[PaigeMiller · Accepted solution]

http://www.theanalysisfactor.com/wacky-hessian-matrix/

[SAS_solution]

Thank you for the detailed explanation! It is so clear, logic and helpful! Thank you!