hello Liton,

since you have tryed the annealing temp. then the quality of your primers is may be the reason. it is better to re-design new primers or check for quality using a software called FAST-PCR (http://primerdigital.com/fastpcr.html). it shows you if your primers are going to form a hairpin (base-pairing to itself) or not. it also will tell you if your primers are self-ligated (anneal to each other) or not