While exploring, I noticed that although my beetle slots were full (6/6), I was still getting beetle encounters that, when I tried to catch them, just bit me and ran away.
This is the same message that you get when your luck is bad and the beetle gets away from you, even if you do have space.
As such, I think it's possible that people might fill up their slots, not check the number of empty slots they have, and thus assume that they were just having an off-day with the beetles, who kept running away.

Conversely, when your territory space is full, the explore lioness encounter tells you that you do not have space for her.

My suggestion is that, like the lionesses, beetles should have a message if you don't have enough space for them.

Something along the lines of,
"You reach to grab the little blob, but suddenly remember that your Beetle Barracks are full. Where will you keep it? The larva takes advantage of your momentary distraction/hesitation to squirm away. Darn it!"


Additional feedback/details:
It could replace the grab button with a lil message like "your beetle barracks are full, you should expand them."
- Xathina (70238)

especially for the text replacing the grab button like with claiming a lioness
- Beetlemaster Caracal (26532)

Maybe as a side suggestion, there can be text saying, "Your bettle barracks are full! Expand them for 1 GB?" and an option to click and expand directly
- Dolce Owl (32000)


Edit: In tandem with this great suggestion, I'm thinking that the beetle encounter could take you to the screen it shows now, with options "grab it!" or "eat!"
You should, reasonably, still be able to eat it, even if you have no space for it. So pressing eat would, as promised, eat it. But if you clicked grab it, it would take you to the message I mentioned above, about not having space, as opposed to just replacing the buttons with the message like the current claiming system does.

Support/feedback is always welcome ^__^