DSF

by preheat i mean let the oven keep heating for about 10-20mins after it reach the temp you want (200 in your case).

I too had this problem and the problem was I wasn't let the oven heat up enough. As soon as it reached temperature I started the timer. Us hi is right, you need to let the oven stay at temperature for at least 10-15 mins before putting in the mugs. Then let the over reach temp again and start the timer. Once I did this they have been perfect every time .

Andy

try mugs in normal position. bottoms down. if they are all up heating my not be able to circulate properly.