When the D3DLOCK_DISCARD flag is used, internally a new buffer is created to make sure there’s no contention between the GPU and the app that’s trying to write into the buffer.
If you’re using really large vertex buffers, and calling Lock() often, it could be that a limit is being hit, preventing new buffers from being created. One thing to try is allocating a circular queue of buffers and not using D3DLOCK_DISCARD.
In general it’s not good for performance to update so much vertex buffer data so often, so it would be better to could avoid doing this if possible.
If this is for animation, you may want to do skinning on the GPU, which wouldn’t require updating every vertex for every draw call.
If you don’t need to update all of the vertices, then it’s better to not lock the whole buffer and use the D3DLOCK_NOOVERWRITE flag.
This article explains it (Using Dynamic Vertex and Index Buffers, Usage Style 2)