I love love love this app. So much fun and great for passing time.
Im not having any issues with killing molerats, radscorpions nor death claws as many other reviewers commented. You do need to KILL the molerats and radscorpions before they burrow into another room. If your vault dwellers dont jump up with excitement when the pest disappears, then you didnt kill it. It went to another room.
Something thats helped me tons with my vault door is to have dwellers level 18+, equip them with the best weapons. Raiders and ghouls NEVER get past my two guards. Death claws always do. Once the DC moves to next room, I switch out weapons on those dwellers in the room to highest level. Swapping out the weapons helps best when battling inside pests.
I do have a bug where sometimes dragging my dweller to another room they get stuck as a big green silhouette and wont move. No other issues.
The more you play and level up your dwellers, the faster you earn caps and improve the vault.