Why is it that the virkids wont go past badass and super badass

I've been farming trying to kill vermivorous and everytime I try, all the virkids go to badass and sba but not ultimae badass or vermivorous, I've got him to spawn once in caustic caverns by accident, but all I got were 4 whites, 3 greens, 3 blues, and a purple. I saw on one page it had a guaranteed chance to drop a legendary on the first kill. If someone can please help me and tell me what's wrong ill be very happy,

In order to make Vermivorous spawn, you'll need to understand how Varkid evolution works. Most of the information you'll need to know is contained here, but here's a quick summary:

• When a varkid spawns or evolves, there's a set chance on whether it'll evolve into the next stage. This chance decreases as a given varkid evolves, but scales with the number of players. More players means higher evolution chances.
• Varkids will only evolve within a certain time limit after entering combat (provided in the link). If the time limit passes and the varkid hasn't evolved yet, it cannot evolve further and must be killed.
• Vermivorous cannot spawn in Normal Mode. You must be in TVHM or UVHM in order to have a chance of getting it to spawn.

Special notes for if you have the Son of Crawmerax DLC: Wam-Bam Island runs on slightly different rules than the rest of the game in regards to varkid evolution.

• Tropical varkids count as Adult Varkids for the purpose of the evolution tree, but they can also evolve into three different elemental variants. If they evolve into one of the elemental variants, kill them, as they cannot evolve further.
• If you manage to make a Badass Varkid evolve while in Wam-Bam Island, it will skip the Super Badass phase and go straight to the Ultimate Badass phase.
• The main questline of this DLC involves injecting varkid pods with Mordecai's beer to make tracking Bloodhound Varkids that cannot evolve. However, if you ignore the mission objectives, varkids will spawn infinitely, making the start of the DLC the best place to farm for Vermivorous.