Problem with Matrices and FloatBuffers

Newbie & Debugging Questions
#1

So I can’t seem to fix my rendering code in lwjgl. I’ve narrowed the problem down to how I’m loading a Matrix4f into glUniformMatrix4() as a FloatBuffer.
Long story short, I know:

public FloatBuffer toBuffer(Matrix4f mat){
        	    	FloatBuffer matrixBuffer = BufferUtils.createFloatBuffer(16);
        	    	matrixBuffer.clear();
        	    	mat.store(matrixBuffer);
        	    	matrixBuffer.flip();
        	    	return matrixBuffer;    	
         }

Does not return a buffer that renders properly (black screen).
And I know that:

private final static FloatBuffer direct = BufferUtils.createFloatBuffer(16);
        
        public FloatBuffer toBuffer() {
                direct.clear();
                direct.put(matrix); // matrix is a float[16]
                direct.flip();
                return direct;
        }

works.
Why? What is different here? Is there a more convenient way to upload a Matrix4f to lwjgl than my first snippet or an easy way to convert a Marix4f to a float[]?
I’m really stumped. Thanks.