Slot structures always reduces the gain. Add a high dielectric layer at a height of half wave length (2.4 or 3.5 GHz) on your microstrip patch. A high dielectric substrate will converge all the field and results in the improvement in gain. this technique is known as adding superstrate layer. you can fine many paper on superstrates in IEEE x plorer.

there is a limitation in choosing substrate,because my antenna must be designed for wearable application,so i have chosen jeans as substrate that have 1.71 relative permittivity,

For wearable antenna case, making antenna with slot gives a compact design with cost of reduction in gain. Instead one can use multiple resonator i.e. a parasitic resonator near to main feed element this will give dual band operation without effecting gain. An optimized design can give compact shape too.

By changing slot dimensions on the radiating patch, some improvement in gain, at lower frequency, may be obtained

u must check if the cutting the slots is making the current cancellation due to flow of the current in opposite direction. Current cancellation will reduce the gain even though impedance matching is good. Check the current distribution and place them so that the current doenot flow in opposite direction.