Withstanding PvE Mobs (Icecrown)

Hey there, I'm experiencing the problem I'm talking about here for quite a while now and as noone in the Trade-Chat could give a proper advice and tickets are taking so goddamn long here, I'm going to ask the community in this forum straight away: What can I do if PvE mobs (regardless of the level, it happens in both old and "new" instances, in raids as in 5-Player-Dungeons) withstand my spells every now and then. Is that a common issue on this server or am I doing something wrong? I've got more than 18% hit but if that attribute wasn't high enough I'd miss instead of mobs withstanding my casts, right? I've played on many servers (also on blizzard servers) and never changed my play-style because in all the time I didn't encounter such problems.

kind regards

Fortunately in Wrath of the Lich King there aren't many bosses that have high resistance to some types of damage (at least it never posed a problem for people doing Wotlk content). If a boss or even the entire instance has high resistance, you should consider changing your playstyle, talents or even entire spec.
There is a reason nobody was playing Fire Mage during Molten Core progression on retail, everything in there has high fire resistance.
Another thing, creatures cannot block or parry if you are behind them, so that could be another reason your attack gets avoided.

World of Warcraft is all about constantly learning about your class and your enemies. Even now in Legion when resistance doesn't really matter people change their talents on a boss by boss basis to be more efficient.

First of all: I'm Shadowpriest. Second: I've got that problem with almost every mob I'm encountering. It happens everywhere: in ICC, in TOC5 but also in older instances like Steamvault. It's not like I'm saying I'm a god so I can't learn anything new but in all these years I'm playing this class now I've never had problems like that. Neither on other private-servers nor on blizzardrealms. There's simply no Priest-talent to circumvent that problem.