I just rectified this situation. I have a thin set of patagonia capilene 3 as a base layer and could reach my valves no problem. With arctic undergarments, I could reach my right valve fine, the left was almost impossible, maybe 2/10 on the first try.
There are a few different things that could be affecting your valves and you need to see which combination is causing the issue.
In my case, I am insanely head heavy in doubles I was using 3lbs in a trim pouch with Hollis F1s and was still face planting. So we moved the bands all the way up, used last hole on the plate, the usual. In the end, what happen was even though I was now in trim and stable I discovered I couldn't reach my valve when I switched undergarments later in the season.
I ended up adding 5lbs to the trim weight, moving the bands a hair down, and using the middle hole on the plate rather than the bottom hole. These changes (allowed by increasing the trim weight since all of them make you more head heavy) place the valve directly behind my ear and easy to reach in my CF200 with heavy undergarments.
The things you need to look at (in no particular order)
1. Tank Bands--You might need to adjust the bands and move them further down the tank thereby raising the position of the tank on your back. I had mine exactly at (or just a hair past) the break when I moved them down to correct my issue.
2. Harness Placement--Which hole are you using for your backplate? Using the bottom hole moves the tanks further up your back, using the top hole moves them further down.
3. Your straps on the harness are too loose or uneven causing the tank to fall too far down your back or just slightly too far onto one side. Make sure the straps are equal on both sides and not too loose. You should be able to get three fingers between you and your harness when wearing your drysuit and undergarments
4. Is your drysuit too smaller? This one can exist for some people who picked up used drysuits or gained a lot of weight. There is not much that can be done about this one, but since you can reach them with thinner undergarments I think there are other things we need to look at first.
It could be one of these factors or a combination of several. You will need to do few test dives and mix and match to find the combination that you like. You may also need a trim weight, but that is hard to say without seeing the setup directly and see you in the water to begin with.
---------- Post added October 9th, 2015 at 07:38 AM ----------
4th Element undergarments are top of the line and I love mine. That said, there are other issues to resolve before buying new undergarments--and I am not even sure that will fix the problem.
