In the case of S block element we will see the difference in sizeof cation and anion i.e. solubility is directly proportional to difference in size

As here,alkali metal fluoride so LiF,NaF,KF,RbF,CsF

So here F is fixed ( fluorine is so small so we have to choose larger cation for higher solubility) then we will see the size of cation ,on going down the group size of cation increases so difference also increases so solubility increases

Some exceptions are their

carbonate,oxide, bicarbonate from the alkali metal and {oxide,sulphide and halide of Be(berilium) here Be is common cation }from the alkaline earth metal.

These can solve by the concept of fajans rule and rule :-

smaller cation ,larger anion (other than common element ) increases the covalent character and decrease solubility so here carbonate anion is common and then we will see for cation smaller the cation less the solubility so solubility increases the group.