Sometimes the body is perfectly relaxed and the mind is willing to drift into a state of active silence.
Some other times the body is sleepy or teh mind is restless and ends up influencing the body.
It happens often randomly.
My main tip is shower or exercise before meditation, that usually advantageous.
Showers always help me as well. Opens up the pours and water harmonizes the body. At times of being sleepy it is not even good to meditate because it can get u in the habit of nodding off in meditation.
I have found that very quiet nature sounds or new age type of music really helps as well as incense to focus the mind on the breath.