The most common issue is adding the custom field to the incident form but forgetting to create the custom field in the probsummary table which is used by Incident/IM. Hence, no place for SM to store the value of the customr field entered into the form.

Thank you for the reply Jason.

Our issue is that, the custom field did hold values previously.

Some tickets doesn't update the value and if we try multiple times, then few of them do update that.

This issue is occurring randomly.

You could try running the rad debugger and display the value of your custom field and run some steps like Save or etc etc and see if this custom value in memory disappear after certain steps. Rad debugger syntax is d <custom field name> in $file.
There could be some steps that trigger background process that wipe the custom field so when you click on Save, there's no more custom value to be saved. If that is what is happening, you need to log a case with Support as they will need a trace to confirm that.
Screenshot below is using rad debugger to display the value of name field in the $file variable which hold the operator record in memory before committing to save. What you see on screen sometimes may not be the same as what will be saved to the db.

Thank you Jason for your assistance.