alan.yurkovic , See the manual link#1 for installation/parts diagram and installation of head. Unless plate/part is worn in the spline area or you are not tightening head sufficiently or using a incompatible head assy., it should not be coming apart. Pop into a local small engine repair shop and have them take a look if problem persist, I do not think they would charge any fee. Also see the 2nd link for further instruction on tightening/installation of trimmer head. Good luck. I hope this helped you out, if so let me know by pressing the helpful button.
http://www.echo-usa.com/getattachment/7011c2f4-035a-47cb-82e5-cf7038faee8e/SRM-225es_1112_022014_2
https://www.youtube.com/watch?v=H7CU44OfYIs