Need some help
Over the last week i've been trying to get my 400m swim time done 1 session i did didn't put much effort in and done a PB next session i really went for it and felt like i was flying but was just over 1min slower. So i then done another swim more relaxed and was close to my PB and then again really went for it and was miles off. I don't understand it as soon as i put in any effort it slows down. I've had someone watch my teachnique and it looks ok. How can i swim faster everything is telling me to get my arms moving faster and put more power through the water.