item.LastModified doesn't always work for me

I have some code in a form’s PostModeChange subroutine that get’s the lastModified date of a four richtextfields I have in the form and put these in some variables. I then have code in the form’s PostSave even that compares the data in those variables with the lastModified date from the rich text fields after the document has been saved. The problem is that this works like a charm when I try it on my local, but when I put the code on a server, it works sometimes and sometimes it doesn’t work. I have a field for each richtextfield, that holds the lastUpdated information and I see that even when I remove an attachment and replace it with another one, the lastModified field doesn’t get updated.

Can anyone give me any clues on what am I missing? Let me know if I should give you more info.

Thanks in advance!