fish_4_all,
I don't know if you have figured out your issue yet or not, but I typed up a little something for you and whoever else might be interested in the info I have to share.
I learned how to do all of my fly fishing in northern Michigan. Out here (I am a soon-to-be Oregon resident) we fish streamers after heavy rains for big brown trout with heavy sink tip lines. I do have a couple pointers for you as well as a couple questions regarding your sink tip set up.
I'll start with the questions. What grain is your sink tip and at what fps does it sink? If it is a really heavy sink tip, it is IMPERATIVE that you get your anchor points set right. A lighter sink tip you can get away with not being so precise but, lighter sink tips won't roll cast as far. My other question regards to your equipment setup. Do you have an actual sink tip line or is it a sink tip attached to a floating line. This can make a huge difference because if you have only 10 or so feet of sink tip on a floating line you are going to have one h$*# of a time trying to roll cast it with 25' of line out.
Now a couple pointers I might like to add. Your anchor points are crucial. If you are unsure what those are, they are the points where the sink tip meets the water and where the weight is distributed throughout the rod. if you have a lot of the weight of the sink tip in the water and try to roll cast you are going to experience an immense amount of water drag (resulting in the 6 feet of messy line in front of you). It also can be disastrous the other way as well. All the weight in the rod and none in the water it will just do a quick flip and you'll end up with yet another short cast. The moral of this explanation really boils down to simple physics. You have to figure out how the weight needs to be distributed FOR YOUR SETUP (each set of equipment is different) that will result in the most efficient transfer of energy from the roll cast motion to the weight in the line. For most of my sink tip setup's, it works best for me this way: for a 30' head of 300 grain sink tip and I will let out a bunch of line to begin with (the 30' head and maybe 10 or so feet of running line). I will let out 20 or so feet of the sink tip and with the rod I will do short little roll casts to position it out in front of me with my rod parallel to the surface. I will then raise my rod tip in a smooth but firm motion so that the line will ski on the top of the water to about the 1 o'clock position. You will notice that at this position you will have only about 5' of the end of your sink tip actually in the water and the rest will bow up to your rod tip. This is called forming a "D-Loop". From this position you will have created a large amount of potential energy in the line and in your rod and will have very little water drag to deal with. At this point you will do what my buddy likes to refer as "chop wood". In other words, in one fast but smooth motion, snap your rod tip in a motion similar to the way a judge smacks his hammer towards the surface of the water. You have just completed a roll cast. It takes practice! Being a good roll caster with a sink tip line does not come over night and there are few fly anglers who are great at this. Regardless, I hope this helps with your dilemma, and you can apply a lot of this to a floating line situation as well!!!
Tight lines!
Joe