Can't Write XMP Files

Question

I'm using Windows XP and LR 1.3.1 and I can't seem to write a specific folder of files. I'm trying to create the XMP files so I can do a global edit to modify the Lense type. When I select all and then click on Save Metadata to Files, the program does not write the files for this specific folder. I have tried this on other files and I don't get the same problem. I have tried moving the specific files themselves to their own collection and trying the same process above, but I get the same result.
These are Pentax K10D files saved as PEF files.

Answer 1

The only way that I could get Lightroom to write the xmp files for these specific files was to make a change to all of them AND I had to turn on Automatically write changes to xmp files in the catalog preferences section. If this option was not set, even after making a change to the files, Lightroom would not create the xmp files.
So I finally got the program to create the xmp files, I just can't understand what's different about these 93 files that insisted on having xmp writes turned on globally before any xmp files could be created. I turned off this global setting after creating the needed files.
The whole purpose of this exercise was to change the Lense Description to a Sigma 10-20mm F4 from a Tamron lense. I found a shareware program that allowed me to select a directory of files and make a global edit to all 93 xmp files at once. I then read the XMP files back into Lightroom and everything is right with the world.
Is the option of NOT changing the lense designation within Lightroom a XMP limitation or is this a design decision of the LR engineering team? It sure would be nice to have the option to PAINT the changes (in this case lense description) from one image to the rest of the group, if one chose to do this.