20-30 is great for a first flush! Congrats! I only saw like 10 with my first flush. :)
Wait two weeks and try again. You gotta give the stones time to de-clump.
Read the other posts in this forum - there are tons of things you can do in between flushes to maximize your next flush. I guess I should write up a text doc so I can copy and paste - my wrists are tired right now and I've answered this issue so many times LOL! If you check my other posts you'll get some good advice. :)