Hello!
each element atom has a certain number of protons, which weigh 1 AMU, but the same element can have a different number of neutrons (weighing 1 AMU), which results in what is called an isotope
Normally, the atoms contain the same amount on neutrons. However, sometimes they gain more neutrons, which increases the atomic mass by 1 AMU for each additional proton.
I hope this helps, and have a nice day!