A disclaimer : I never have the luxury of owning a DJI. Hence my reply is not based on experience of flying a DJI.
RE : the boys bought me propeller guards since I'm a beginner, could the extra weight degrade flight time?
No. The weight of 4 prop guards are insignificant. It has no bearing on your flight time issue.
2 factors that may affect flight time :
1. Wind factor. Under strong wind, the drone needs to work harder to maintain flight. Thus battery drain.
2. The Speed Mode. Under your drone's standard speed mode (22mph), you are expected to get 30 min flight time. If you set a faster speed mode, flight time will reduce.
However, it is unacceptable that you got only 15-20 min flight time, no matter what.
RE : I live at 6800' elevation, so could altitude be affecting battery performance?
I have no answer for that.
RE : the battery would not charge at all. I contacted DJI and they replaced it fairly quickly.
Does your DJI come with a charging hub? If yes .......
What did they replace? The battery or the Charging Hub? I would bet it's battery. Obviously they did not replace both.
Maybe the replacement battery is a dud? Maybe the Charging Hub is a dud?
I would call DJI again on your problem.
except from the web :
New batteries are in sleep mode, you must be inserted into the drone to charge and wake up from sleep mode. If a battery in sleep mode is inserted into the charging station, the charging station will not respond.