You might send it th wrong address.
USCIS recently changed the I-765 filing address. See this new I-765 Instructions for new filing address at the following website.
If your EAD is based on pending I-485, you need to file the EAD applicaiton to the USCIS Phoenix or Dallas LockboxfFacilities based on where you live (see: Page 10 in the Instructions):
http://www.uscis.gov/files/form/i-765instr.pdf