Juki f 300 My screw seems too short on presser foot. when i have on walking food it jiggles loose after a very short time.can i get a longer one?

SOURCE: Brother walking foot help, please

Have a look at links below: open a manual of a similar model.
http://www.sewingmanuals.com/
http://shopping.superpages.com/Brother+LS-2125+Mechanical+Sewing~PS_20~BEFID_96329~ty_pr~PID_22126552

SOURCE: Attaching walking foot

you probably purchased the incorrect walking foot for your specific machine. check and make sure if you have a high shank or a low shank....the way to determine if you have a low-shank is to measure the screw from the plate and look at the distance if it is 5/8 then you have a low, any higher about 1in. then you have a high shank, so you would purchase a foot depending on your shank.

SOURCE: How to connect the walking

Turn power switch to "0"
raise the needle to highest position by turning the wheel toward you(counterclockwise) and raise the presser foot lever
release the foot by raising the lever located at the back of the presser foot assembly
place a different presser foot on the needle plate so that the bar on the presser foot is in line with the slot on the shank
lower the presser foot lever and fix the presser foot onto the shank. If the p foot is in the correct location the bar will snap in you should hear a click or snapping sound let me know if this helped thanks! amy

SOURCE: Model 6230- presser foot lifter won't move presser foot

That's the problem seized up use a penetrating oil from the top and work it up and down and it should free up

SOURCE: Can I loosen the presser foot so knits will not stretch

Pressure on the pressure foot can be adjusted on some machines by turning the knob on the top leftside of the machine directly above the needle assembly. Look for a knob that has fabric types on it and adjust accordingly. Also check your thread tensions.