Hi Siva,
There is a criteria to elect a master in IAP,
Master election criteria:
—Step#1: look into IP scope, default-ip can not win
—Step#2: look into 3G capability
—Step#3: look into AP class, IAP-13X gets higher priority
—Step#4: look into uptime, older AP gets higher priority
—Step#5: look into AP MAC, bigger address gets higher priority
To avoid this you can configure preferred master, means whichever IAP you want to make as a Master unconditionally, configure that as a preferred master.
Please feel free for any further help on this.