Good morning

For about 6 months I have been experimenting with etching with Ferric Chloride. The results have ranged from outstabding to truly awful. A consistent problem is the appearance of circular unetched areas which vary in size from less than 0.5mm up to 3mm or so in size. I found yesterday that the etchant should be diluted before use. Is that correct and could using the etchant in concentrated form cause the problem?
Thanks
Alan