Alright...so i had out, Embodiement of Crime and Punishment and one face down trap. He has 3 facedown spell/traps and Summons out Red Nova Dragon. So i activate one of my guys effects"all opponents monsters changed to atk position and must attack if able"...he summons and goes straight to EP
So im thinking "hmmm ok..maybe one of the monsters he used for synchro helped him" no..neither of the 3 read anything liked that.

So his turn comes back again...beginning of round i activate it again...and then out of nowhere he SWITCHES TO DEF MODE!!

Now if im reading Red Nova Dragons card right...it just says hes immune from anythimg that would Destroy his Dragon (spells,traps and monster effects)

Every other time ive used my cards effects, nothing has been destroyed. He should of been forced to attack and take the damage both rounds..what am i missing?
N13 and 31's effect only affect the monster that's present of the field when it activates, from what you said it seems that you use 13 or 31 effect before red nova is summoned so its not affected

and switching the monster to def after its affected by the effect does work to prevent the monster from attacking, since its not possible for most monster to attack in def
Ok...the first part makes sense...i did activate before he was summoned..all good there i suppose

But the second time...BEFORE, at start of round, he switched to DEF mode i activated it.
And N13 and N31s effect says "must attack if able"...so how did he switch to DEF mode.

Shouldn't he be forced to attack..i have used it before and people couldnt not attack and couldnt switch to DEF mode.
N13 and 31 effect forced a monster to attack, if possible, effect like that make a player cannot skip BP and must attack if they control the affected monster in attack pos, it doesnt prevent the monster to change position if they're able to

the other time you used it and people are forced to attack is most likely due to they cannot change the monster's position either because they just summoned the monster this turn or they already change position before the monster are affected by 13 or 31