Are you soaking the line in warm water prior to wrapping it?
Soaking it in warm water and a little dish soap and or fabric softener will relax the line prior to the wrapping process. After a good 10 minute soak, pinch the line between the fingernail on your thumb and index finger and kind of "squeegee" some of the water out of the line. This will get some of the starch out of the new line.
When the knot is finished, hand tighten it the best you can and let it dry. It should tighten up pretty well.
Keeping the line wet before and during the process is key.
After a few turks heads, you will start to figure out how much pressure you will need at the beginning of the knot to end with a tight knot.
For me, If I am starting my third pass and I am not needing to use an aid (fid, marlinspike or scribe [in my case a scribe]), I know my Turkshead will be too loose. I start over.
Its trial and error... and a little help from water/soap.