* Metals tend to lose electrons, not gain them: Electron affinity is the energy change that occurs when an electron is added to a neutral atom in the gaseous state. Metals have low ionization energies, meaning they readily lose electrons to form positive ions (cations). This makes them less likely to gain an electron and form a negative ion (anion).

* The addition of an electron to a metal atom is often energetically unfavorable: When a metal atom gains an electron, the added electron experiences repulsion from the existing electrons. This repulsion makes the process energetically unfavorable, and therefore, the electron affinity value is often negative or very small.

* Experimental difficulties: Measuring electron affinity for metals is challenging. The process is often too complex and requires specialized techniques that are not always available.

In contrast, nonmetals generally have a much stronger tendency to gain electrons. This is because their electron configurations are close to a stable noble gas configuration, and adding an electron fills their outermost shell. This makes the process of electron gain energetically favorable, resulting in positive and readily measurable electron affinity values.

While it's challenging to directly measure electron affinities for metals, theoretical calculations can be used to estimate these values. However, these calculations often have limitations and may not accurately reflect the experimental reality.