There are several factors that could cause thread not to trim all the time. Some possible causes include:

1. Incorrect threading: If the thread is not properly threaded through the machine, it may not be able to trim properly.

2. Dull blades: The blades that trim the thread may be dull or damaged, preventing them from cutting the thread cleanly.

3. Machine malfunction: There could be a problem with the machine itself, such as a malfunctioning sensor or electrical issue, that is preventing it from properly trimming the thread.

4. Thread tension: The tension of the thread may be too loose or too tight, which can cause the thread to bunch or not lay flat, resulting in improper trimming.

5. Thread quality: The quality of the thread may be poor, causing it to be too thick or too thin, which can affect the trimming process.

It is important to troubleshoot and fix the problem, otherwise, it could cause issues with the final product, and also it could damage the machine.