Windows server 2016 hyper-v "ran out of memory"

I have several servers with windows server 2016 and all with the same hardware. They runs different type of vm with Hyper-v. In every server i allocate up to 56 gb, leaving the last 8 for the os. Now my problem is that, in some Servers, i can't do that. If i used 48Gb of ram for the vm and i try to add other 8 then the Hyper-v give me the error message "ran out of memory" with code error 0x8007000E. If i try to add another VM with just 4Gb it works fine, but then trying to add another one with 4Gb gives me the message error. ironicaly, one of the machine that gives me the problem has 10gb of free Ram, while one machine that i filled with 56Gb of VM, before the last vm of 8gb Ram, it had less space than this and yet i could create this last VM. I already tryed with RamMap to clean the standby memory but it didn't work. I can't restart because my servers goes 24h. Any idea? Also i hope it's understandable what i asked, my english is really poor i'm sorry.