I don't have a Juki but if it's a lever, you may need to add a drop of sewing machine oil to the joint. Joints can freeze up from accumulated gunk, but oil can soften it up. You usually have to wait a day for the oil to penetrate, and it may take more than one application.
I found this service manual for the machine, but didn't find a diagram of the feed dog lever:
https://www.juki.co.jp/household_en/...e%20manual.pdf
If there is a spring involved with the lever, it could be that the spring has worn out and needs to be replaced. However, if the lever is frozen in place, I would think the problem is more likely to be lubrication.