How hard should i try to get addresses?

Unfortunately, tracking down addresses just comes with the territory. I have to admit, I have no idea what postable is - I am assuming they are getting an email? Some people might think it was spam at first, some might be leery of sending info through a site they don't recognize, so that could be part of why some people aren't immediately responding (totally guessing here). Emails often get lost or overlooked, so some probably just didn't realize what it was. I hate to say it, but it's probably a glimpse ahead to tracking down RSVPs (every couple unfortunately finds themselves in this position as well), so if they are guests who you don't even really want there enough to follow up one-on-one with them (i.e. personal text or phone call), I would definitely revisit why you're inviting them in the first place? Good luck!

Alot of email will go to the spam filter automatically if someone doesn't communicate with you on a regular basis. It's how technology works.
If you wouldn't miss them not attending, why invite them in the first place? Your guest list should only be those you can't imagine the day without, not randoms to please the parents.

I used Postable and texted the link individually to everyone (no email). When sending them the link, I told them that if they didn't want to put their info into the site for whatever reason, to just text me back personally. A few of my older guests did that.

I had to check in with a few people who I know just forgot. "Hey! I want to send you some mail soon, can you share your address with me? I didn't see it come through yet. Thank you!"

One person didn't give me hers even though I knew she kept meaning to, so she just didn't get a save the date, lol. I told her verbally and she had sent us an engagement gift and everything - means well, but pretty forgetful. She recently gave me her address - just in time to get a change of plans card.

Ugh I hated getting addresses. The worst part was I would message friends and family to get addresses for ones I didn’t already have. Sent out save the dates, was super relieved when wedding invites were being prepared because I didn’t need to gather addresses again. EXCEPT a bunch of the people I had messaged 7 months prior had moved again and their mail wasn’t forwarded. So I had to completely redo their invites, and pay the 1.25 to mail them again. (Oversized and overweight invites.) The first time around I had no problem getting addresses. The second time around, the only problem was my fiancé’s half sister and adult nieces. They moved so often and a month before the wedding invites went out they decided to move several states away for absolutely no reason. We are pretty sure they are running from responsibilities (debt, eviction, etc.) Fiancé messaged them to get their new address and they would read his messages but ignore him. He decided to cut them off because they just use his family when it’s convenient. We stopped trying and uninvited them to the wedding.

I did something similar but I messaged a lot of family/ friends directly via social media or text. I gave it a couple months ahead of time and waited till most people had responded and those who didn’t didn’t get a save the date. At this point we’ve pretty much set our guest list as those who have filled out the link. Lol but I really think it’s up to you and your own comfort level. You technically did your part

I would only try a postable link once. Anyone who didn’t answer, I’d track down using a different method. But, track down only who you actually want to invite to your wedding...if they’re not that important to you, don’t bother. I wouldn’t re-send a postable link as I’d presume there was some reason it didn’t work for them (they tried but the website didn’t submit properly, the link got sent to spam, they received the link but presumed it was spam and didn’t open it, they didn’t feel comfortable with the 3rd party website but forgot or didn’t know how to reach out to you directly, etc). So, I’d give them a second chance by an alternative means.