I use my Charge 3 with an Android phone, but there are similarities in the connecting and use of a smart phone GPS with your tracker. It sounds like you have created the correct profiles for exercises with which you can use GPS. I remember on older Android phones there was an accuracy setting for the GPS which provided a lower sample rate and coarser plotting in favor of battery life. You might check your phone's GPS settings, and if there is an adjustment make sure it is set for high accuracy. When you start your exercise there is that period of time where the tracker is syncing up with the phone's GPS. I have found that a quick sync with the phone's Fitbit app beforehand and then leave the app open in the background will almost always ensure a good connection. Lastly, depending on the construction of your phone and any kind of external protective case you might have it in, you might try facing the phone screen away from your body if you have it in a pocket. This might help with the reliability of GPS signal reception.
