More and more of your flags are getting declined, and that's leading Stack Overflow to trust your flagging skills less. One way that's manifested is in the reduced number of flags you're allowed each day. Once upon a time, you were able to see a quantitative measure of that trust, called "flag weight", but no moreno more (and good riddance!).
Review your recent flags (from your profile page, click the link to the right of the "helpful flags" label), and reflect on why they're getting declined.