Snipers Mark / Mark on Hit not working with frostbite on hit

My Snipers Mark is suddenly not triggering on hit, I have several spent hours researching this issue and in every single other previous case the explanation seems to be a mana issue. In my case howeveer, that cannot be.

* Vorici -7 Mana craft on both rings.
* Linked Inspiration (also tried lifetap) on all my attack skills
* 850 total Mana (I have all of my auras turned off)
* Mana leech and mana regen (my mana bar stays at 100% when I cast)

The 4 link setup is Snipers Mark (20/20) --> Mark on Hit (20/20) --> Inspiration (4/0)--> Vaal Rain of arrows (20/0).

Ascendency is Ranger/Deadeye including the Focal Point ascendancy node. I have tried using lower-level skill gems also, that didn't help. I tried linking just the snipers mark and mark on hit by themselves on a different pair of boots... also no result.

Finally i removed my frostbite on hit ring and snipers mark came back. While it is still not a mana issue there seems to be an issue where frosbite on hit and mark on hit cannot work together... Bug?

Can you apply more than 1 curse at a time?

Like DoubleU said, you need to get +1 Curselimit to do that. (Either from the skill tree upper right or from an item like hunter chest, corrupted amulet or similar.)
Frostbite is a hex and Sniper mark is a mark, but both count as curses.