There are any number of things that could cause the problem. Look in your status window for any error msgs that mite give you a hint where the problem is.
try checking your settings in mIRC's options (view/options or hold down your alt key and click the o key) under dcc and under dcc/ignore, as well as under other/lock. You could also have a script interfering/putting the sender on ignore.
Remember that the problem could be on the senders end and not yours. Have you tried with more than one friend? In the event you are trying to download from bots/strangers, thats not a safe thing to do, please read this thread