Sounds like you fall under the Western Regional Supplement. Other posters from that area can help you.
Are you part-time?
You are formally in the Union when you have paid your Initiation Fee in full, and are up-to-date on your monthly dues.
You work under the Contract's provisions from Day One.
Maybe you are just laid-off for a week or so until volume picks-up.
Here's a quote from an old Western Supplement I have. . . .
ARTICLE 4- SENIORITY
SECTION 1- PROBATIONARY PERIOD/TRIAL PERIOD
(c) - PROBATIONARY PERIOD (PART-TIME)
A new part-time employee shall attain seniority when he/she has worked seventy (70) days within a six (6) consecutive month period. Prior to attaining seniority, as defined in this Section, the employee shall be considered a probationary employee and may be discharged without such discharge being subject to the grievance procedure. However, the Employer shall not discharge or otherwise discipline a probationary employee for purposes of evading the terms of this provision or to discriminate against Union members. Upon completion of the probationary period, the employee shall be given a seniority date as of his/her first day worked within such six (6) month period.