My juki hzl-f 300 won't pick up the bobbin thread and the…

Hi, my name is Martyn.

I am sorry you are having these issues today, it can be frustrating.

I am happy to try and help work with you to try to get this resolved if I can.

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I am in SE Australia

Try a sharp new needle first (and check it is in correctly, flat side to the rear for modern machines, or as per Manual )... this is often enough to resolve skipped stitches

Did you have a recent Jam ??

Check for rough patches and burrs everywhere the thread passes... use a fine emery board to polish

Tease out a QTip/Cotton bud etc to be a little fluffy, then trace all over the metal and plastic regions and the bobbin case… it will likely catch and show you where the burr is

NOTE : If it is under warranty, do NOT attempt to do too much, lest you void the warranty
If you have had a recent jam, then this suggests timing may be affected

In any event..... here's how to check

Unplug machine and remove all thread

Remove Needleplate, Free Arm and Bobbin Case so you can watch the needle easily

In correct Timing, the needle will be at the lowest point just before the tip of the hook passes behind (to catch the top thread)

If the hook passes too soon or too late, it will need re-timing...

This short VIDEO shows how a stitch is formed… if the hook is not in the correct position, it will not catch the needle thread

https://www.youtube.com/watch?v=ML8CMNzW6Tg

Here is a basics guide to know when and how, and whether it is time to hand it off to a trained Tech

https://www.youtube.com/watch?v=ZsebAKZxmU0

If the needle to hook is out of balance it will never form a stitch
As needle rises, the hook MUST be above the eye approx 1.8mm
Turn slowly by hand and watch the relationship
​Is there any deviation to this balance of needle to hook ??

If a thorough clean of the top tension unit, bobbin area and hook race.... plus using a new sharp needle (inserted with the flat side to the back) does not help, then your Hook to Needle timing is out.

Timing is not really something that can easily be achieved without training and, particularly in the case of a novice,...access to the service manual. The tolerances are quite fine in 3 dimensions and any error can cause more troubles and expense.