Wild Guess

In most tales games side quests have a certain space limit, meaning that after a certain event occurs after the event you start a side-quest, you can never start that sub-quest or finish it. Case in point, the Presea's Childhood Friends side quest. I managed to get a new-new Key Crest for Presea, however, I went to Meltokio, taked to the woman staring at the window and it hasn't triggered. Either I need a full party or it's too late.

You need a full party for mostly everything.

I remember doing most sidequests only after passing through the gate at Vinheim, iirc, which means there shouldn't be a cutoff point.

There might be a few here and there.

I see. Well I'm in Sylvarant now so the instant I get back I'm gonna pay that woman a visit.

If you just went to Sylvarant for the first time after going to Tethe'alla, then it is not too late for that side quest. There is a side quest in Sybak you will want to start soon though. I think the side quest in Sybak I am referring to is miss-able if you don't start it before you make pacts with all the summon spirits.

I always make pacts with Undine, Efreet and Sylph before going to Tethe'alla for the first time.

It has never cut me off from any sidequests.

Or did you mean all summons?

I meant all summons. I was able to do the Presea's childhood friends side quest after I made pacts with all the summon spirits, but I think I missed the Joshua and Rosa side quest.

Ah, I see.

Regarding Joshua and Rosa. If you answer incorrectly you cut the sidequest short as well.