I've heard of other people having difficulty to cast summons because they have absorb spell effects (like atronach or stuff like that). If I remember well, your own summon spell is aimed at yourself and can be absorbed that way. I hope they fix this soon...
Atronach Stone + the level 100 Alteration 30% absorption perk = 80% total. That suddenly makes sense. Early in the game I had 0 absorption, then got stone, so 50% then atronach so 80%... so my spells don't cast and it got worse over time. Of course healing spells, alteration spells, etc. don't have this problem. It also explains why my magic fills back up after casting it. I would switch to third person and see if I see the absorption 'waves' that come off your character but my character also disappears for a split second when I cast summons, whether they cast or not. I'm not sure if this is it, but it would explain it, and it did get harder to cast about the time I took the atronach perk. However it also doesn't effect bound weapons, and seems it would.
For some of the other theories. I've been casting spells for a long time, I think I know how. Tried casting them at my feet, tried casting them every way imaginable. And with my first character who was a mage I remember standing on high surfaces and casting summons down a floor below me without problems. I can not do that now even though I have a the perk for longer summon range. I remember summoning in combat no problems. I would never try that now, I keep summons with me 24/7 so that I know they are ready for combat.... of course this often means I spend a minute at a time while walking casting the same spell over and over. Then the noise alerts enemies and defeats the purpose. By now I already need to cast it again.