There may be the following reasons for the lock of the CNC vertical lathe tool holder not being tight:

1. Improper adjustment of the knife holder: The knife holder may not be properly adjusted in place, causing the lock nut to not hold the knife tightly. Before using the CNC vertical lathe, make sure to adjust the position of the tool holder correctly, and make corresponding adjustments and fixes according to the user manual or operation guide.
2. Damaged or loose lock nut: The lock nut may be damaged or loose, making it impossible to effectively lock the tool holder. Check the condition of the lock nut to make sure it is intact and use the proper tool to secure the nut securely to the tool holder.
3. Lack of lubrication: friction between the tool holder and the lock nut may cause difficulty in locking. Make sure to add the right amount of lubricant in the right place to reduce friction and improve locking.
4. Worn or damaged parts: The parts of the tool holder and lock nut may be worn or damaged, which will affect the locking effect. Inspect key components of the toolholder and locknut, such as threads, springs, and washers, to ensure they are not damaged and replace if necessary.
5. Improper operation: The user may not apply enough force correctly and evenly when locking the knife holder, resulting in the knife holder not being securely locked. Make sure to apply proper force evenly when locking the tool holder and follow the correct sequence and steps.

If you encounter the problem that the CNC vertical lathe tool holder is not locked tightly, it is recommended to carry out the above inspection and adjustment first. If the problem persists, it is best to contact the manufacturer or maintenance personnel of the CNC vertical lathe for more in-depth inspection and repair. They will have more specialized knowledge and experience to help solve the problem. Please be sure to follow the relevant safety regulations and operating guidelines when operating the machine tool.