Depends, not enough info.
What was the temperature? Were you taxiing with Mixture full rich or leaned?
30 seconds may not always be long enough to get rid of whatever is causing it.
Do you have a reliable EGT gauge?
**** Disclaimer, the following is based on personal experience and shared on an anonymous forum ****
I have had cases where I had to run the engine at full power with the mixture leaned to around 1500-1550 F for a long as 5 minutes till whatever was fouling the plug (usually a little lead ) was spat out the exhaust pipe.
Alternatively you may have either one of a dozen possibilities ranging from a broken lead to a disconnected ground to an internal mag failure.