there are no 'sensors' as such , its done by rotational differences detected thru the ABS setup..
have you pressed and held the button til it 'boings' ?
you are 2.5% out in rolling radius (2 on front versus 2 on back) which the car may detect as a low pressure .........
but its should be within tolerance not to trigger it.
Better to run the same size all round though.
try a few miles then reset again.
or in the process of fitting the tyres a sensors been damaged is an outside possibility too
John
edit : bacillus just posted the same thing