I've had this problem as well-with DDX try upping the temp to 24c and/or the developer strength. Now I use Rayco UFG (from Rayco Reveals booklet) for 16 -18 mins at 24c 1+1 -it's great stuff and virtually identical to FX 10-see the Darkroom/Film Dev Cookbook for this. Speedibrews Celer-Mono is similar and obtainable from Silverprint.