I've run some 5k's with the Dex and have never had a problem. In your case, if the receiver is too far from the sensor (doesn't sound like it was) then you won't get a ??? you will get the no signal indicator.
To the OP, my guess is consistent with some of the comments above...your sensor is moving around during the run. Hopefully you can sort this out.
thanks everyone--sensor is on abdomen; it isnt consistent-- this past weekend did a lengthy walk/run with the problem-- this mornnng there was no problem and this morning I am using a sensor into its 3rd week. If I find a solution I will let everyone know
I actually have this problem all the time. Pretty much every time I run or work out I either have ??? or the out of range symbol. It is so frustrating for me because it is the most critical time to have information about your glucose level. I have tried working with Dexcom on the issue, but we haven't found a solution yet. I am doing a half marathon at the end of May and I don't see any reason to even bother carrying the Dexcom because it's worthless to me. I have been on it for about 9 months (switched from the Minimed CGM) and I am completely disappointed in it. I am getting close to quitting using the Dexcom altogether.