Andy I think you went a little too light - I come from the 10% school, 10 % less, 10% more.  There is more debate about hvy shot light shot training and than you could imagine. The E. Germans and there usual propganda reported that their sub jr.s trained with the jr weight ( not really sure what the kilo's were). Their theroy was the body would make the mechanically changes to accomocdate the heavier weight. Between my Soph and Junior years in College I trained several weeks/months with an 18 lb shot. The results were very good. Hapy's point about relaxing because you have a very light weight is I think the issue. Another ex. hate to use this but my son started throwing the 8 lb shot last summer age 10, he should be using a 6 according to USA T/F the 8 clearly too heavier. About a month ago I bought a 6 lb for him and the technically gains are pretty amazing.  He has at age 11 the basics of the full glide down. 

So maybe go a little heavier, ( 17/18)  learn the Scottish spin progression and commit to it.  Break out the hippo shuffle as needed.