Hi Barryl,
Your question could be the best answered with "it depends".
At layer 2 each unicast frame thats send must be acknowledge with a (ACK) frame, if the ACK is not received a frame must be re-transmit.
Re-transmission affact the WLAN in two ways, it increase airtime consumption and it affect the throughput.
Re-transmissions are normal and always seen. Most data applications in WiFi network can handle a layer 2 re-transmission rate up to 10%. For Voice applications is this 2%, Exceeding a 20% retry rate will almost always impact performance.
Root causes
1. Number one reason is RF interference from some sort of interferance device,802.11 or non 802.11 like a bluethoot, microwave, etc.
2. Number two reason is Low SNR, the distance between signal strength and the noise floor, keep at lease 25dB space in between.
3. Adjecent Channel inteference
4. Hidden Node problems occur when one client station's transmissions are heard by the access point but are not heard by any or all of the other clients stations in the basic service set (BSS). This can give collisions within the 802.11 BSS that can cause re-transmissions.
5. Mismatches Power settings between accesspoints and clients.As a client moves to the outer edges of the coverage cell, the client can "hear" the AP, however the AP cannor "hear" the client.
6. Multipath issues can cause intersymbol interference (ISI) which cause data corruption. Because of the different in time between the primairy signal and the refelected signals.
7. Client or AP driver issues
As you see there are a lot possible reasons for re-transmissions and hard to answer with a single answer. From your picture i see the you have lot of re-transmissions on the 2,4Ghz radio, and less at 5Ghz. Because 2,4Ghz is very busy those days it seems like you have some kind of interference.
Some tips. Turn of 2,4Ghz if you can. Lower your transmit power to arount 30mw (15dBm) or equal to the lowerst transmit power of your clients. Disable the lower datarate below 24mbit. And do a site survey with a proffesional tool like a Ekahua sidekick spectrum analyser by a certified engineer.