Questor - that's a tough one, both in position and question. I would have to stand behind you when you were shooting to see if, or what, you might be doing something wrong. Double D asked if your rifle was at 45 degrees to your body? I would ask if you were using your sling and how you used it, if you had something to pad your left elbow (assuming you are a right hand shooter), if you had a rest, if you had your right leg braced - toe into the ground, if you were anxious about your shot, and most prolly a whole buncha other questions.
I like the prone position sometimes - it helps if the grass is cut or the snow isn't so deep so that I can see what I'm shooting at, if the bugs aren't flying about, and yada, yada, yada. I have a funny story about that I might let out sometime. Some fellas just aren't comfy with a heavy recoiling rifle and the prone position. The 270 will snap on ya and may not be all that comfortable to shoot prone. And, let's not forget that if you have been pounding yourself for a while with that 270 you might not be 'loose' enough to shoot well by the time you go prone.
It's sort of like starting off a range day with the easiest shooting rifle you have and gradually going uphill to the most difficult and hardest recoiling rifle you can shoot. By the time you get there, if you are anything at all like I am, you are gonna be real nervous and jerky just bringing the rifle to bear, much less waiting for it to go off. If I start the day with my 6.5 Swede, then go to an 8mm, then a plain stocked 06 and try to finish up with a short barrelled 444, I can guarantee ya that my scores with the 06 will be pretty bad and I don't even want to think about pulling the trigger on the 444. If I reverse the process my scores are much better.
Also, you might be suffering shooter's fatigue and just might not be aware of it. It might help to make certain your shooting matt/ground cover is comfortable and that your support arm and rifle are supported by a backpack - that helps steady you and the rifle and may help.
You may recall from your service days that the foxholes you shot from were bermed sufficiently to enable you to rest the rifle, or yourself, for better accuracy - you need a similar setup for shooting prone, or at least I do. You may also remember from any experiences you have had or from reading hunting articles that just about everyone I've read advocates any kind of a supportive rest when you are not shooting from the bench - same applies here. Hope this helps. Mikey.