How do I get all subscribers in one segment?

So, our situation is a little different than most. Our business hasn’t taken advantage of email marketing until recently. Upon signing up for Klaviyo, we had a list of about 40k emails that we brought over. We had recently installed a spin-the-wheel type pop up through an app via Shopify that got subscribers. We then replaced that with a pop-up through Klaviyo on our website for people to subscribe. Our integration is with Shopify. What we are having trouble with is that we are trying to have one segment that includes ALL of our subscribers and that is obviously continually updating as more people subscribe. We have tried to create campaigns that include everyone that has subscribed within the last 30 days/90 days/6mo/etc—but the system doesn't seem to pull those numbers accurately, because if we look at our metrics, we can see that over 300 people have subscribed in the last month—but our segment only shows about 100. We are having difficulty figuring out why it wont pull these numbers when creating a campaign. Is there are way to create a segment that includes everyone who has given us their email/subscribed (and continually updates)? I have looked everyone online and can’t seem to find an answer to this question that can relate to our unique situation. Any help would be greatly appreciated!