Here are a few things to check:

1. Verified Purchaser: Ensure that the reviews you're trying to include are from verified purchasers. The Review quote block can only display reviews submitted by verified purchasers.

2. Filter Settings: Double-check your filter settings. By default, only 4- and 5-star reviews appear in the selection modal, but you can adjust this filter to include other ratings if needed. How to feature reviews in emails with the review quote block | Klaviyo Help Center

3. Review Settings: Make sure that no review elements are hidden from your reviews list. Go to the Reviews tab in Klaviyo, select Reviews settings, click Onsite widgets, choose Product reviews, and check if any elements are hidden. 

4. Recent Reviews: Sometimes, newly submitted reviews might not show up immediately. Ensure that the reviews you're trying to include are not too recent.

If you've checked all these and the issue persists, it might be best to reach out to Klaviyo support for further assistance. They can help troubleshoot and resolve any underlying issues.