/cas/validate returns no

[behconsci]

may be dumb question, but my tickets are always invalid ("no"), if I validate it right after getting in the client app:

myapp.com ---> sso.mydomain.com?service=myapp.com ---> myapp.com?ticket=ST-LALA. In this step 3, I try to validate at /cas/validate/?service=myapp.com&ticket=ST-LALA. Then if ticket is valid, then I save it to localstorage. This is my plan.

but validation gives me always "no" and I see in sso backend, the ticket is consumed right away. am I doing something wrong in this flow?