Ahhh... yes. You can't start with the ice water. I thought I detailed that in this thread.... hmmm.
We ran some math on it and found that the 15g HLT is not enough cooling capacity, as you have found. What I have done to get around this is to set everything up as follows:
Fill the HLT with ice (like normal).
Hook up all connection like normal except the return to the HLT. Keep the hose running into the HLT as you circulate the ice water and collect the initial hot water into a bucket for later clean up. Continue this until the wort is below 120ºish (should be 3-4 minutes) then connect the HLT return and circulate ice water to chill down to ale or lager temps. This has worked very well for me (the only difference with my current setup is I have a CFC now instead of a second coil, but that should be changing shortly after the new year!)
I hope that helps and I apologize for not making it clear. Woop.