The chain is
usually very tight with "M" setting and this seems to be a factory setting.
Then once it became loose, people turn the tensioner to "H", but interestingly, most of people found their
chain wasn't loose enough for "H" setting. (i.e. "H" was too tight.)
I just found our member Leonarhodo experienced the same thing recently. (see below.)
http://leonarhodo.exblog.jp/5765489#5765489_1
If the secondary (lower) chain tension is really loose and "H" position doesn't help it, then you might need to remove a link from the chain as Oleg suggested, or replace the chain itself.
You can always ask your A-bike distributer to supply the chain.
Alternatively, replace it by your self.
The chain is called
"#25" and used by minimote, pocket bike, etc.
Search for "#25 chain" under "sporting goods" category to find the listing on eBay UK.
There are some minimoto parts retailers who offer
chain cutter and half-link for #25. I had a url of one of these, but cannot find... sorry. If you google "#25 chain", you should be able to find some more info on this.
#25 chain has two different thickness. one is normal #25, and another is thicker, heavier
#25H (H stands for High durability or something like that.) I guess A-bike uses
normal #25.