Chronic troubles consistently covering the holes? Bliffy notes? Headaches?
As a whistle player, it sounds like you’re doing everything right. 
No doubt some kind folks will chime in here with great tips on how to play with piper’s grip. Here are a few more generic things to consider:
We learn by doing, which unfolds over three broad stages.
In the first stage, it’s a struggle to simply do the skill.
In the second stage, it’s hard to do it well.
In the third stage, it’s hard to do it well consistently. You’ll know when you’re finally in this stage because it’s the interminable one. 
Also, learning any physical skill is like gymnastics. No matter how well the gymnast does the quadruple twisting layout plutz move in the air, the judges score her way down if she doesn’t stick the landing. And it turns out that sticking the landing is often harder than that mind-boggling move in the air. The same applies to learning to play whistle. For example, once you learn how to do rolls, the real trick is in how to play them without mangling the notes afterward and interrupting the flow of the tune. You can diagnose the problem(s) yourself. It may require extra, focused practice on what your fingers are doing immediately after the roll, releasing any tension in your hands (and lips, and neck, and…), paying close attention to the pitch of the next note or two, the duration, timing, and attack of those notes, your finger placement and movement to get those notes, your use of air after the roll, whether and where to catch a breath, and so on.
One of the keys to “sticking the landing” is to sing the tune in your head, especially through the tricky spots. In other words, you know that critical voice in your head? It’s shouting, “HERE COMES TROUBLE! GLIDE INTO THAT HALF-HOLE WITH T2! OOPS! MISSED IT! NOW WIGGLE B3 FOR THE VIBRATO! FASTER!” So replace it with a calm, easy voice that’s doing nothing more than lilting the tune. Dee-aye-dum-diddle-dum… The idea is to quit thinking about the mechanics and how “difficult” they are and instead simply let yourself play the tune.
If things are still going awry, it may help to watch yourself in a mirror. Ask yourself what the problem specifically is and sift through as many causes as you can imagine: fingers straying from position? Why? Do your forearms or elbows flap around? Does the angle of your fingers change? Are you shifting from covering a hole with a fleshy part to landing on a crease and not-quite-sealing it? Often the quickest way to isolate and solve a problem is to slow waaaay down. So sloooooooooow that you hear and feel every detail, nothing escapes your attention.
Finally, thinking about “playing position” on the whistle…once you have a hold and posture that enables you to play effortlessly, without tension, and it sounds good, then your hands and fingers should always play from this hold and posture. Except when they don’t. That’s less of a contradiction than it sounds. The idea is to use a consistent hold and consistent finger movements, but also realize that these are not completely static. Know what “home base” feels like and play there most of the time, but flit around as necessary, and then land back at home base. That’s a skill in itself, worth some focused practice.
If you stall and need help, find a good whistle player to help diagnose. Or post a video here and let the hive mind sort it out.