Hi and welcome to Just answer.
This will likely be a lubrication issue. If not serviced within the last 2 years, it may well be due anyway, but if you wish to try yourself, a TINY amount of machine oil on all the moving parts will help....this will require opening the top sides and bottom for access. To specifically deal with the reverse, you will need to find the mechanism by moving the 'stitch length knob/reverse' to ident which things are involved. It may not come good first application, so be a little patient and persistent.
Hope this information will be helpful.
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I'm just following up with you to see how everything is going. Did my answer help?
Let me know,Chester