By default, Mastodon prevents you seeing a boosted post again until 40 other posts have flowed through your timeline. There is more info at:
➡️ https://fedi.tips/how-do-i-stop-seeing-the-same-boosted-post-again-and-again-in-mastodon
The 40 number is hardcoded into the Mastodon server software and currently cannot be altered. It seems to have worked better in the days when Mastodon was much quieter.
If you're comfortable using Github, you can vote for this number to be adjustable at https://github.com/mastodon/mastodon/issues/18693
