Sometimes there are some spans/transactions that are missing from my dashboard in Sentry. I can see them being sent in the debug logs, and I can see that Sentry is returning 200 in the network tab. They also don't correspond to the numbers in my stats page.

There is a chance that those spans/transactions are being dropped due to Retention Priorities. You can disable or configure them by going to your Project Settings > Performance > Retention Priorities. To test if that is the case, you can turn all parts of this feature off, and see if you start accepting more of the spans/transactions you were missing.

There is an ongoing feature request to show when spans/transactions are dropped due to this feature in the stats page here.