You're not supposed to fit every single thing in a monster's entry into one fight with a monster. That's the whole point. Otherwise there are no themed sets of encounters, because you blew all the interesting stuff on the very first fight - the whole party knows the Yechzui can fly, use webs, use web blasts, have an eyebeam attack they can seemingly only use once, hidden extra claw arms, and improved grab and great grapple now. So clearing out the Yechzui nest just lost any possibility of any surprises from the Yechzui themselves, unless you go the 'suspension of disbelief? What's that?' 4e route and have different Yechzui with different powers that all look the same.
You gain nothing, lose the opportunity to show more abilities of the enemies in different situations (later, the party flee from 5 Yechzui, getting across a chasm using levitation, and all duck behind cover - why? They know about Yechzui eyebeams. You just lost a great 'gotcha!' moment, where the party thinks it's safe, but no, no it isn't! Or deprived the wizard of a chance to use his Knowledge (Dungeoneering) skill to warn the party about the Yechzui's ranged capabilities), and suffer ALL the downsides of long samey rounds of combat i.e. boredom, endless dice rolling, loss of momentum.
Rookie, whatever, none of that has to do with anything. If things are not changing, your players get bored. Sure, different groups can survive different amounts of boredom. Some will happily roll d20s to attack the same goblins forever. Most won't. Combat in DnD takes ages. Making it take longer than like 3 rounds, fighting the same enemies, is generally beyond the boredom threshold of most groups. Either the terrain has to change (running battle over the roofs reaching a dead end, party detaching the giant mill wheel and fighting on it as it rolls down a hill, pirates of the Caribbean style), the enemies have to change (the noise alerted the guards - the Vampire Queen and her attendants are coming to deal with the interlopers personally!), or some other dynamic of the fight needs to change to keep it interesting.
Things being the same for too long = boring. Like if there are no clues in an investigation, and it goes nowhere for three hours, people get bored. Combat is no different than any other part of GMing in that regard.