Have you really installed the drivers given by the manufacturer of your bt-card instead of those WIN are given?
Windows is causing many issues regarding bt.
As far you see, the problem is not caused by the speaker, working properly with other devices. Its WINDOOFS (sic)
Give this a try…
Click Start Menu and type ‘sound’ to go to the sound control panel
right click on the troubled bluetooth sound playback device and select properties
go to the ‘Advanced’ tab and uncheck 'Allow applications to take exclusive control of this device
Another alternative if that doesn’t work is to troubleshoot Bluetooth device, this may fix the issue for you.
Hello,
I was looking for the same problem and it was really hard to find the solution, although nearly all bluetooth speakers were affected.
You have to turn off the “absolute volume” in the registry:
https://support.microsoft.com/en-us/help/14169/windows-10-fix-bluetooth-problems
It’s the registry part in the last section " Troubleshoot problems with Bluetooth accessories" in “Audio”.
You can also change the value of “DisableAbsoluteVolume” manually into 1.
(Path: HKLM\SYSTEM\ControlSet001\Control\Bluetooth\Audio\AVRCP\CT)
Restart and enjoy normal volume control again. 
Best Regards!
Great! This is quite a help for WIN users.
Thank you, though I am not using it.
Spot-on, worked perfectly!
Worked great. Thanks!
Thanks Kakulukiam, that worked for me.
Worked perfectly! Thanks.
I was looking for it, have to sign in for a thanks.
Really Appriciated!
Just spent 5 minutes making an account to simply say thank you for this fix!!
I don’t know where the registry is. Where can I find it?
Press the Windows button and type “regedit”, than press enter.
The famous Windows - button!
When taking a look at the keyboard I found it.
That is showing us how MS is influencing the market.
Where is my LINUX-button ?
Work’s great. Tnx.
Work like a charm, thanks
Indeed the fix with the absolute volume works. Only I noticed that I can increase the volume on my Soundcore boost a lot. I think it’s even a bit dangerous for the speaker.
May be dangerous for your ears and neighbors! 
I had exactly this problem. Crazy how common it seems to be. Disabling “absolute Bluetooth volume” fixed it, and it’s very easy to do. Takes two minutes and requires no specialist knowledge.
I wrote a very quick three-step guide in case it’s of use to anyone else:
Thats MS, not a soundcore/ANKER problem. 
Be cautious if you have 2 same speakers paired in stereo mode, I tried to change the registry for “DisableAbsoluteVolume” to 1 in stereo mode to have volume control via Windows. Unfortunately, changing “DisableAbsoluteVolume” to 1 seems will mess up the stereo link between 2 speakers and causing 1 of the speaker (Master, the one linked direct to audio device) cannot be turned off by its own power off button, end up had to turn off the speaker using Soundcore App.
Once changing the “DisableAbsoluteVolume” back to 0, redo setup again of the 2 speakers in stereo mode, everything is back to normal, and the stereo link works without issue.
Therefore, if you have 2 speakers in stereo mode, changing registry of “DisableAbsoluteVolume” is potentially a risky move, be prepared to have some hassles to bring the speakers back to normal stereo mode.
PS: Speaker model is Soundcore Boost (upgraded)