$pp$ collisions can produce neutralinos and charginos directly via electroweak production. This can occur via $s$-channel $\gamma$, $Z$ or $W$, e.g.,
$$
q\bar{q} \to Z \to \chi^0_i \chi^0_j,
$$
which is a Yukawa Z-Zino-Higgsino interaction (and is significant if $i\ne j$), or via $t$-channel squarks, which is a $q$-$\tilde{q}$-Zino interaction.
You might be able to boost these diagrams by enhancing the Higgsino or electroweakino components of the lightest neutralino and chargino, or, of course, by decreasing the masses of the supersymmetric particles.
Alternatively, $pp$ collisions can produce neutralinos and charginos via cascade decays of squarks and gluinos (QCD production). From my memory, whilst prospino calculates the production cross sections at NLO for squarks and gluinos, you must decay the particles by calculating their branching ratios with a separate program (see http://arxiv.org/abs/hep-ph/0609292).
I think, then, that you are looking only at electroweak production, rather than QCD production. Could you be clearer about the behaviour you observe? Which MSSM parameter do you increase? What is its effect on the masses and composition of the lightest neutralino or chargino?
A shot in the dark: maybe if you are increasing the scalar masses $m_0$, this results in increased $\mu$ via EWSB conditions, decreasing the Higgsino component of the lightest neutralino and chargino, and enhancing $t$-channel electroweak production?
See "Neutralino pair production at proton-proton collider" http://arxiv.org/abs/hep-ph/0003292 for a fuller discussion
