CD$_4$ is cryocondensed on three substrates : oxized tantalum, gold and platinum, with thicknesses from 1 to 12 monolayers (ML). When a few ML of CD$_4$ are deposited on Pt cleaned by heating, dissociative electron attachment (DEA) is strongly inhibited, contrary to Ta or Au contaminated substrates. As the thickness increases, we observe that resonance peaks broaden and their maximum moves to higher electron energies. Both peak area and maximum anion yields tend to a plateau. Theoretically, at low surface charge ratio, the total electron-film potential is repulsive starting from the third layer; total collision analysis for both the desorbed ions and incident electrons explains the experimental peak behaviors. Adsorption energies indicate that condensation of CD$_4$ and residual water on Pt or Ta is mutually exclusive. Inhibition of DEA for small CD4 thicknesses on Pt are explained as well as the significant desorption yields from water contaminated substrates.