Here are the things I've found work best with the Combi Tank.
1) I can't figure out if you're using the slot guides when you load film. If you don't have them then you need to get some from HP Marketing. 2) The film has a tendency to dislodge during agitation if the retaining clip is on too tight. The clip should only slide down until it contacts the film. Otherwise the film is distorted and the chemical movement will push it into the next slot.
3) Because of the slow fill and drain the tank is best used with development times longer than 8 minutes. Agitate for the first minute after the tank is filled and then for ten seconds every minute after. I start the timer when I BEGIN to fill the tank and don't start to drain until after the timer has gone off.
4) To best avoid agitation marks face each sheet of film AWAY from the center spine. The spine will cause a linear mark if the inner sheets of film are facing towards it.
5) Wear gloves.
I've been using the tank for about four years and get very dependable results. Like every development technique you will need to refine your times to get your desired results.