The search for hidden phase using photo-induced phase transition is an important challenge in multistable materials. In this work we report the study of Prussian blue analogs of formula RbxMn[Fe(CN)6](x+2)/3. zH2O. For composition of Rb+ above 65 %, a thermal hysteresis loop is observed as a reflect of the electron-transfer phase transition from MnIII (S=5/2)-NC-FeIII (S=1/2) at high temperature (HT phase) to MnIII (S=2)-NC-FeII (S=0) at low temperature (LT phase). At low temperature the HT phase can be thermally quenched up to the T(TIESST) temperature (TIESST = Thermally-Induced Excited Spin-State Trapping) above which it relaxes back to the LT phase. Upon the decrease of Rb+ amount, the thermal hysteresis and the T(TIESST) are getting closer and overlap for values of x below 65 %. For the latter compounds, the phase transition is inhibited and the hysteresis loop is hidden. By this simple consideration we explain the observation of Light-Induced Phase Collapse already reported giving also some trends to obtain new hidden phases.