Off-axis distortion

I noticed a problem with my new glasses:- things looked out of focus
when I was looking downwards. If I tilted them (top away from my face,
bottom towards it) that improved it a lot. I bent the angle between the
arms and frame as much as seemed reasonable, and it's improved the
vision in the bottom portion of the lenses, but at the expense of vision
looking upwards, which is now quite distorted, and the downwards vision
still isn't as good as my old glasses. I also noticed that the effect
seems more pronounced in one eye than the other.

I already had the lenses changed because I knew they weren't right and
they were giving me headaches, but I couldn't work out what was wrong,
so I downgraded the lens material from 1.6 to 1.5 plastic thinking it
might be due to too much dispersion (although I've had 1.6 before which
were OK).

What are likely causes? I'm guessing it's mainly because the lenses are
too flat, but I wondered seeing as it's worse in one eye than the other,
could it be they're miscorrecting my astigmatism, or would the
differences between each eye's prescription cause a slight difference in
the experience of too-flat lenses? The prescription is -6.25 -0.75 10
and -6.75 -0.5 37, and it's the first of those which seems a tad worse.
The only differences from my old prescription are that the axes have
changed from 15 and 42.5.

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TH * http://www.realh.co.uk