How to improve the communication distance with Gazell?

My gazell configuration is as follows:

#define PIPE_NUM					0
#define	TIMESLOT_PERIOD				2000
#define	TIMESLOT_PER_CHANNEL		2
#define	CHANNEL_TABLE_SIZE			1
#define CHANNEL_TABLE				{18}
#define	SYNC_LIFETIME 			(3 * CHANNEL_TABLE_SIZE * TIMESLOT_PER_CHANNEL)
#define	MAX_TX_ATTEMPTS				3
#define TX_POWER			NRF_GZLL_TX_POWER_4_DBM
#define TX_DATARATE			NRF_GZLL_DATARATE_1MBIT

Payload size: 22bytes

frames per second: 100

When the communication distance up to 2 meters, the device often cannot receive the ACK from host, or the host cannot receive the packet form device. I don't know that is normal or not? Can I improve the communication distance by setting up a more reasonable configuration?