There is something that is blocking ventilation to your hybrid battery causing it to over-heat. Therefore, it is not switching to EV mode and causing load on engine leading to poorer fuel economy. Check to see if there is a loud noise coming from the battery while the car is running. If the noise is loud, the battery is over heating causing the fan to work much faster. Plus the hot weather would also lead to poor fuel economy as the battery will overheat and the car will start to minimize battery usage to help prevent damage to the battery. Remove any things that might be blocking the battery vents. This is a common problems in hybrids during summer, especially when the temperature is over 43 degrees.
Also, do not use any other oil than recommended in the owner's manual. Using anything different will lead to poorer fuel economy.
My Civic hybrid was doing the same but then I found that there were some pages that were blocking the battery ventilation vents near the rear headrests. I removed them and my fuel economy went from 13 KPL to 16 KPL (in city with full time AC in peak rush hour traffic). The car also started to use the battery(battery assist) more frequently. I am using Total 5W30 since the last 43,000KMs and the fuel economy has never gone below 14 KPL.