My Janome is only sometimes collecting the bobbin thread…

Hi, I am Martyn, welcome to JustAnswer, 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.

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

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 ??