Mine typically runs around 300F but that can change with wind & air temperature.
Your temperature will drop some with chicken due to the moisture dripping onto the coals so a strong bed of starting coals is important. If you start with an incomplete lit bed the drippings from the chicken can prevent the unlit coals from lighting causing lower temps and longer cooking times.
How did your bed of coals look right before you put your chicken on? Here is a picture of my bed of coals before I started some ribs...
See how they are white hot all the way across the top layer. The 20 minutes is a guide. If you notice the coals are not completely started across the top layer give them a little more time before you start cooking. These coals were lit with lighter fluid BTW.
You can also bump open the air shutter a bit to allow more air to the coals. Also removing a rebar will increase temps like 50 degrees. I like to do that when not cooking a full load of chicken. I leave both out when cooking chicken legs in the grate.
Just remember you want a strong bed of coals when cooking chicken due to the moisture content of the bird dripping onto the coals. This action and distance of chicken from the bed of coals is what helps the PBC work it's flavor magic on chicken IMHO.