Some enemies have a boost to Mag Protection and/or Mag Resistance.
In the case of some Arachnos, they have BOTH. Which means not only does it take stacked holds, but the holds wear off quicker.
By the way, were you playing on Villainous (difficulty 1)? Because the only wolf spider unit I can think of (which I forget their exact name) is the Boss. Think it's Huntsman or something? And it would be "down-graded" to lt value while retaining it's hold protection and resistance.
Damage Proc Mini-FAQ
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Are you sure it's hitting? Do you see a damage tick on the mob?
I have an Ice/Energy Dom and I typically hold a LT on the first try with Block of Ice.
Ya, I am on Dif 1...So far, that wolf spider was the only one to get hit by block of ice, take damage, get slowed, but not be frozen...I think your right about the resists...Thanks.
I hate all this terrorist business. I used to love the days when you could look at an unattended bag on a train or bus and think to yourself.... I'm going to take that.
Yah, it was probably a scaled down Boss Huntsman type that has mez protection and mez resist then. 
Them and Fortunata's will be your bane at low levels as they both take two holds and have resistance to mez's to lower the duration.
Damage Proc Mini-FAQ
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Ok, I am using a lock of Ice at a Lieut wolf spider...nothing hapens...I cast it 4 more times...nothig happens....Sure, it slowed it...for a moment...Its a Mag 3...I thought Mag 3's if cast 2 times could hold a boss? And just one use would hold a Lieut...But 4 casts in a row and nothing but barely slowed for a moment???
I hate all this terrorist business. I used to love the days when you could look at an unattended bag on a train or bus and think to yourself.... I'm going to take that.