Firstly, try to receive off of 2/3 other people to make sure it's definitely your end which is the problem. If it works with other people, it's not your fault it's not working.
Otherwise, the most common problems that I find are:
1) You have an IP mismatch. Type
/localinfo -u in mIRC and then ask someone to send you a file or initiate a DCC chat session with you.
2) If you have Windows XP you could have the Personal Firewall turned on without even knowing it. Even if you think it's disabled then you might be wrong :P - check
these instructions.
If these two suggestions do not apply or do not work, unload any scripts related to DCC and type
/remote off, then try again. If you have installed any "speed patches" which are supposed to make your DCC sessions go faster, uninstall them.
If you're on a network (school, work, cyber cafe, home etc) then you could have a firewall there without even knowing it. Talk to whoever sets up the network and deals with all the techy stuff and see if they can sort it.
A number of DCC issues are dealt with at
this site.
You may also want to try a
Search for "DCC receive" etc - expand to "All Forums" and "All Posts" for best results.
G'luck, and merry christmas

Regards,