tic tac toe

Outputs dont need to be, but anything that is read should be (ie in an if statement or on the RHS of an assignment). So, as far as I can, row_position and C are missing from the sensitivity list (and watch your simulation hit an infinite loop because counters shouldnt be in this process), because you are reading them to update themselves:

row_position <= row_position + 1;

Obviously this only applies to signals. Variables and constants cannot go in a sensitivity list.

In the checking process, even though its commented out (its very bad practice to leave commented code lying around - at least give a comment as to why its been commented out), you are missing D_in, memory_last_row, row_position and changed_chk from the sensitivty list.