Is there any brown "rust" on it or crud? Try blowing it off really well with a powerhead. If there is "rust" (diatoms or dinos) on it, gently take your fingers and wipe it off. Sometimes they get diatoms on them and this causes the polyps to retract and they have trouble coming out again because the opening is all 'gooed-up'. You could also try moving it to a slightly higher flow area if possible.
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