It is possible that the starter pulley is not connected to the pawl and the crankshaft. Therefore the engine doesn't turn over when you pull the starter cord. Make sure the pulley and the pawl are engaging.
Remove the bolts and lift off the starter and separate the starter from the trimmer.
Make sure the recoil pulley and the pawl are engaging with each other, that they are not broken, and make sure the recoil pulley has enough spring tension when you pull the starter cord.
Hold the starter in place and pull lightly the cord to make sure the starter engages in the pawl. Tighten the screws and check that the starter works.