Unfortunately, Greek verbs don't behave as predictably as the learner might want...
While the aorist often
can be derived from the present stem by adding a sigma, this isn't always the case. There are some verbs which use a different stem, and verbs whose stem ends in a liquid (λ,μ,ν,ρ) lose the sigma.
There was a discussion not too long ago about this here
, you might find it has some additional information which is helpful.