It’s a little difficult to offer advice with the lack of details.
When you say the kiln can’t get past 400 degrees I am assuming 400F
not 400C. This suggests to me that the problem is either with the
heating elements or possibly the energy controller if one is fitted.
Obviously the heating elements are not fully burnt out or it wouldn’t
heat at all. Similarly if the problem is the relay it wouldn’t heat
at all because no power would be supplied to the elements. You can
check whether the elements are heating by switching the kiln on and
bushing in the safety interlock switch if one is fitted. If the
elements are heating properly they should be obviously red hot. If
one section of the elements is glowing and the other isn’t then
either the element is burnt out or has a loose connection (less
likely but worth checking).
Another possibility is the energy controller. These work by
switching the elements on and off to control the energy input to the
kiln. When these are set to their maximum setting elements should be
powered continuously. On a lower setting they will switch on and off.
Ste this to full and check if the elements are on continuously.
Finally the issue could be the temperature controller if one is
fitted. This should leave the elements switched on until the
temperature reaches the setting on the controller. If this is not
working properly it could be the electronics in the controller or the
thermocouple probe to which it is connected which measures the kiln
All the best