  Background: Following the 2007 precise measurements of monopole strengths in
tin isotopes, there has been a continuous theoretical effort to obtain a
precise description of the experimental results. Up to now, there is no
satisfactory explanation of why the tin nuclei appear to be significantly
softer than 208Pb.
  Purpose: We determine the influence of finite-range and separable pairing
interactions on monopole strength functions in semi-magic nuclei.
  Methods: We employ self-consistently the Quasiparticle Random Phase
Approximation on top of spherical Hartree-Fock-Bogolyubov solutions. We use the
Arnoldi method to solve the linear-response problem with pairing.
  Results: We found that the difference between centroids of Giant Monopole
Resonances measured in lead and tin (about 1 MeV) always turns out to be
overestimated by about 100%. We also found that the volume incompressibility,
obtained by adjusting the liquid-drop expression to microscopic results, is
significantly larger than the infinite-matter incompressibility.
  Conclusions: The zero-range and separable pairing forces cannot induce
modifications of monopole strength functions in tin to match experimental data.
