I think I have more than needed Activity and Merits to be a Sr. Member... why is it not being updated?

You meet the Activity requirement yes, but you are falling short in Merit. You need 250 merit to be a Sr. Member and as of now, you only have 56.
I see... well looks like it's going to take me forever to get promoted since it is hard to get merits. Can you perhaps suggest me how I can get more merits? Thank you in advance.

I think that merits don't have to be actively sought, they will be earned naturally and spontaneously if you post quality responses that add to the interest of the conversation.
I think it's pretty smart that merits are not too easy to accumulate quickly, it really helps to make it credible and you know pretty quickly what kind of posters you are responding to when you see their merit/activity ratio