Please help with tough chicken. I have a delicious recipe for Chicken Divan. It calls for boneless ?

They are to be baked in the oven @325 for a half hour, then placed in the cerole dish under the sauce and cheese for another 30 min. This is the 2nd time I have made this and each time, the chicken that looked so beautiful, was tough to chew. When I make other chicken, it is always tender. Can anyone help me with why this is happening. Thank you.