SQL Server Not Consuming all Memory - Insufficient System Memory in Resource Pool 'internal'
Essentially, the error is saying that there is not enough memory available to Sql Server for it to grow its internal buffers - this generally happens if Windows is having to page a lot or other processes are consuming the memory.

You should also remember that just because you tell Sql Server to use a minimum amount of RAM, it can use less if the host OS needs it more - the host always takes priority and the buffer pool will get trimmed if the host starts to detect memory pressure. For instance, in the documentation for Server Memory Configuration Options it says:


  SQL Server is not guaranteed to allocate the amount of memory specified in min server memory. If the load on the server never requires allocating the amount of memory specified in min server memory, SQL Server will run with less memory.


I would check to see what other processes are running (for instance Service Broker), and how much strain they are putting on the server. If possible, increase the size of the VM or move some of the other processes onto other boxes.

Lock Pages In Memory Option

It is a common misconception that when you use Lock Pages In Memory that it can never be released. Using the LPIM setting prevents Windows moving pages in memory to virtual memory on disk (the page file). It does not however, prevent Sql Server from reducing its memory footprint if it detects that the host OS is low on memory. The snippet below is taken from the same page the previous snippet is taken from:


  Setting this option does not affect SQL Server dynamic memory management, allowing it to expand or shrink at the request of other memory clerks.


Sql Server uses the Memory Notification API's within Microsoft Windows to detect when the host is running low on system resources. If the host is running low, Sql Server will begin to trim the buffer pool in response. This is an important fact - it is Sql Server that is releasing the memory in response to an external event - Windows is not taking it from Sql Server. The below is taken from the Memory Management Architecture Guide:


  When SQL Server is using memory dynamically, it queries the system periodically to determine the amount of free memory. Maintaining this free memory prevents the operating system (OS) from paging. If less memory is free, SQL Server releases memory to the OS.