is your date set correctly in the settings. A lot of times, this is caused because the call comes in this year, and the phones settings reset to 1970ish. Go in, and manually reset the date, then get a friend to call you to see if it records the date.
You can also have a setting that only keeps call logs for a day or so. make sure that is set to a timeframe you prefer. (my phone is set to 30 days)