Decreasing Solubility as Solvent Moves Higher in Paper Chromatography
The solubility of the solutes is the same at all points in the chromatography process. The solutes are clustered together at the beginning (at the solvent front) because none of them has moved very far yet. The reason for the separation of the solutes isn't that they become less soluble, it's that the solutes are moving at different speeds, like cars in an auto race:

  _______
 /____/SF\___
 |_,._____,._)
---`'-----`'--     
  _______
 /____/S1\___
 |_,._____,._)
---`'-----`'--
  _______
 /____/S2\___
 |_,._____,._)
---`'-----`'--


At the beginning, both cars (solutes) S1 and S2 are even with the solute front car (SF) because the race (chromatography) has just begun. However, over time, the SF car outpaces S1 and S2 because it is moving faster.

                     _______
                    /____/SF\___
                    |_,._____,._)
                   ---`'-----`'--     
         _______
        /____/S1\___
        |_,._____,._)
       ---`'-----`'--
              _______
             /____/S2\___
             |_,._____,._)
             ---`'-----`'--


And at the end, you get great separation:

                                                                _______
                                                               /____/SF\___
                                                               |_,._____,._)
                                                              ---`'-----`'--     
                        _______
                       /____/S1\___
                       |_,._____,._)
                      ---`'-----`'--
                                           _______
                                          /____/S2\___
                                          |_,._____,._)
                                         ---`'-----`'--


(Thanks to retrojunkies.com for the ASCII car)