On failed concentration check you would have big red text "casting failed" (or something like that).
Quicken (and possibly some other metamagic, except maximize and empower) may not work if you set it with "always use: on" (or something like that part 2) instead of using general toggle.
In most cases the thing is that slower spells (especially aoe) will not cast if you are not facing your target anymore (especially when enemy moved left/right too much at close range, because your facing sight is like triangle) or there is object on straight line between you two (even if it was not at the beginning of casting).
Additionally, various statuses may prevent you temporarily from casting (anti magic, silenced, ability damage, negative level) or even interrupt it (trip, held).
Hard target with right mouse button or "next target" key instead of soft auto target or mouse pointing, otherwise many spells (especially slow to cast or aoe) will not land where you want (sometimes because moving target has lost selection for even very short moment when probably code was checking for it).
Keep your camera at proper height by aiming on torso, so not too low (may hit ground in close range) and not too high (may hit somewhere far away).
Above tips should help you handle most problematic spells in general on various classes.
After some practice you will do it automatically with continuous success.
