Have you checked to make sure you have not accidentally switched off the feed dogs? My machine has a switch that you turn to lower the feed dogs for free motion sewing. Check your manual or on-line manual to find yours if you are not familiar with the switch/dial/button.
Also, I have had feed dogs get clogged with fabric lint to the point they did not grab well. Brush them with a toothbrush or machine brush and vacuum to get all the stray lint out.
Good luck. If it is not one of these simple things, you could check with Brother service on-line for other ideas. They answered a query I had on my Brother serger in less than 48 hours and helped me fix the problem I was having.
Is the feed dog properly set? I sew Singer but have read Brother users posts on sewing forums that the drop feed drops a little too easily on some models, and they constantly have to raise the feed.
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