The silica gel is made of SiO2 polymers, where some of the oxygen atoms are negatively charged (or could be bound to a proton). This means polar solvents (as well as polar solutes) will be attracted to the gel and won't move up as quickly. Hence, you need non polar solvents like hexane to develop a TLC.
Increasing the polarity of solvent will increase its intermolecular interactions with the stationary phase and slow the solvent down.
