Hi Rong, yes I have had a similar problems with my juniper and olive trees. After feeding them with BioGold for the first time (in the past I used to feed with liquid fertilizers) the pellets did not wash out quickly and gnats appeared, as I wrote in another post.
At the Bonsai Centre they told me it is quite normal being organic (my soil on the other hand is 100% akadama, so inorganic).
After about 1 month I have now removed the leftovers, the gnats reduced and nothing bad has happened to the trees.
I am now switching back to liquid (little bit stronger, according to the Bonsai Centre's advice) - next year I'll try Clicio's method, wrapping pellets in teabags.