Choice of solvent during thin layer chromatography
Chromatography is really an empirical science. The fluid mechanical side and the adsorption phenomenon is somewhat well understood but choosing mobile phases is essentially based on trials, sadly to date. If someone has given you a ratio, say 8:2, it means someone must have tried it and they found that this mobile phase works the best. The main criterion for choosing the mobile phase is that all the mixture components must be soluble in it. Thus if glycine did not move, this means that the mobile phase has no affinity for this solute (it is perhaps insoluble in it) and it is a poor choice. One then starts to tune the polarity of the solvent mixture. Think about a petroleum sample, which literally has thousands and thousands of different molecules. Now think yourself: will they all dissolve in 100% hexane? You do need an additional solvent.

There is no theoretical restriction in chromatography to use multiple solvents or a single solvent. For example in gas chromatography, you always use a pure gas. The reason for using a mixture of solvents in liquid chromatography originates from the concept of "general elution problem of chromatography" which basically says that there is no single adsorbent or a single mobile phase which can separate everything. Not only separation scientists use a single mixture composition of mobile phase (isocratic), they can also continuously change the mobile phase composition as a function of time. This is called gradient elution. You cannot do that with thin layer chromatography without using fancy stuff.

People are trying to implement machine learning, so that one can enter the molecular structures contained in the mixture, available stationary phases, and with a lot of statistics and tons of experimental data, the computer can tell you to try a certain mobile phase composition.