I'm not entirely sure why it didn't work. Sometimes images are not proper tomographic images, in which case SPM should (I think) give a warning message about not being able to convert them. Either way, I'd suggest looking at the MATLAB window to see if there are any warnings.
It's also possible that Philips is using some features of DICOM that are too new for SPM to support. DICOM is a very large and complicated data format (there are thousands of pages of documentation), so there are sometimes versions that SPM can't deal with. However, there are loads of DICOM converters out there, which may do the job. I'd suggest a search for DICOM at
http://www.idoimaging.com/ . Alyternatively, you could have a look for DICOM to NIfTI converters at
http://www.nitrc.org/ .
Best regards,
-John