Probably a coil stop or other part mis aligned and the plunger is not solidly contacting the coil stop. Make sure your Eos switchnis gapped properly and opening or closing as appropriate. Look at the other flipper and compare.
Also make sure there is some vertical up and down play for the flipper shaft. If there is no play you may be losing energy to binding. Start there and look at the dozens of flipper rebuild info stories that must be documented on the web.