It is generally true that the electron affinity becomes less exothermic down a group, because of the increase in atomic radius.
There is a well-known exception that the electron affinity of Cl is more exothermic than that of F, because fluorine has a high electron density; adding electrons would be unfavorable because of the electron-electron repulsions.
In Group 2 and Group 5, this is more extreme: the electron affinity becomes more exothermic down the group. Why is this so?