Well, i understand your concern. The only time your roots wouldn't be getting enough oxygen is if you've either packed the soil in there like there's no tomorrow.. or you over watered her. You need to saturate the soil when transplanting to give the roots the proper direction to grow (down). Don't make mud. and don't pack the soil in there so it's as hard as a brick.

The water should pack the soil down enough without you having to push on it..
I couldn't say why your plants died a few weeks after you transplanted them.. but if the transplant was the cause of that, it would've died within days.. not weeks.
Remember to water you plant that you're transplanting a day in advance, otherwise your rootball will fall apart when you try it.

Cheers man.. I grew up on your name. Monty Python,Faulty Towers and John Cleese.

Hope this helps. :P