Gluing down veneer

I've been using yellow wood glue to glue down veneer marquetry on MDF boards with mixed results.In a recent project the wet glue caused some of the thin  veneer ((pear wood in this case) to expand, producing large ripples which could not be ironed out or sanded out. The project was ruined. I don't own a vacuum press system. Am I using the wrong glue or possibly too thick a coat? Your thoughts?