Depressed/Bored Ferret

Question
Thanks for your reply, more info:
Yes he lies down flat on the ground on his belly and I know ferrets do it sometimes (my other two do it on occassion) but he does it A LOT with episodes from 5-15 minutes. (Compared with the other ferrets who flatten out for only seconds at a time) I don't think he's out of shape, I let my ferrets run the house for 4-6 hours a day and take them out on walks sometimes and he's very energetic when he is playing and is within the normal ferret weight (he's actually shedding and going through the seasonal male weight loss thing right now, which he's done before). He usually flattens out 1/2 hour - 1 hour after I let them out; sometimes he plays a little bit, but it always seems like he's bored and only perks up if I'm preparing food or going outside. If I pick him up and hold him he wants back down again and goes and lays flat. I can't say, it's very random but happens every day. I was under the impression they do it when they're bored/depressed, is that true? I want my babies to be happy but I don't know what to do for him. Is this something I should worry about and get him checked out by a vet, or are some ferrets just weird/or too smart and get bored easily? Sorry this is kind of a weird question, but I'm worried this doesn't seem like normal ferret behavoiur. Thank you.

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The text above is a follow-up to ...

-----Question-----
Hi I own two male ferrets (Big Ferret and Puppy, ages 3 and .5) and one female (Little Ferret, age 4). Big Ferret I've had for his whole life and almost as long as I've had him he gets depressed. He lays flat on the floor and doesn't move and won't play with me. So I bought him the female as a friend 2 years ago, which seemed to work for a while but he got depressed again. So, another friend, this time a young male to keep him on his toes. Again, same thing. He's not sick, perfectly healthy, he does play and wrestle with the other ferrets but always get bored quickly. I don't know why he does this, I've tried buying him more toys, paying more attention, etc. The other ferrets don't show this behaviour for extended periods of time at all like him. Any suggestions, please help me solve this riddle!
-----Answer-----
I may need a bit more of a description and more details.  Ferrets have a behavior we call "flat ferreting" which is when they stop their activity and lay flat on the ground.  When they need a moment, are tired or whatever, they lay flat for a while.  How often does he do this?  What leads up to it, specifically?  How long does it last?  Ferrets that are out of shape can flatten longer than a young ferret, so?  Get back to me with the answers and I will see if I can be more helpful.

Answer
Ok.  I want to tell you not to worry that he is probably flat ferreting and is probably resting or watching the activity or bored (if boredom is the case search Bob Church online for great enrichment ideas or in Ferrets magazine!).  My concern is that since I can't seem him, maybe it would be worth a vet check up to make sure there isn't something wrong - lethargy can be caused by a number of things including Adrenal Disease.  I hate to be vauge, but I wouldn't want something to happen because I think it is probably a behavior thing and it turns out to be a medical thing.
Let me know how it goes
Ailigh