The grammar isn’t wrong. Two things; 1. 教えて is more like tell/teach me so if you just want them to “say” it again, maybe go with “言って” 2. …ませんか is a little overboard politeness-wise. It kind of sounds like “won’t you please say it again?” – you can just say 〜いただけますか？
Probably don’t need to use such a polite form: すみません、もう一回言ってください
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The grammar isn’t wrong. Two things; 1. 教えて is more like tell/teach me so if you just want them to “say” it again, maybe go with “言って” 2. …ませんか is a little overboard politeness-wise. It kind of sounds like “won’t you please say it again?” – you can just say 〜いただけますか？
Probably don’t need to use such a polite form: すみません、もう一回言ってください
Can drop the が after すみません as well.