South Africa finished fifth in the tournament, which involved the world's top six netball countries.
Fastnet is a shortened version of netball equivalent to Twenty20 cricket.
Quarters are only six minutes long, shooters can score from inside or outside the goal circle, with goals scored from outside earning double points, and rolling substitutions are allowed throughout the match. Each team can also select one quarter as a power play, when all goals scored are doubled.
"We are happy that we moved up a position in the Fastnet tournament, after finishing sixth last year," said Kotze. "We were generally on a part with the other teams until the third quarter, when we began to flag."
Kotze said South Africa had not taken full advantage of the power plays.
"Instead of letting the ball go, we protected it, and then we didn't score enough goals to earn the double points, whereas the other teams used the power plays to increase their scores, and that led to the difference in the scores.
"However, the other teams knew they could not afford not to take us seriously," she added.
"We led England by two goals at one stage, and some of our players were outstanding. What we need, though, is a stronger bench, with 12 players that can keep up with one another. Some of our better players were over-played because we could not afford to rest them."
Erin Burger, who was Player-of-the-Tournament at the World Netball Championships in Singapore earlier this year, was twice named Player-of-the-Match, and Kotze said that goalshooter Chrisna Bootha had been the talk of the tournament. "She had a fantastic tournament and has made her mark at international level," said Kotze. "Top players from the other teams came up to her and commended her. Amanda [Mynhardt] once again showed her strength both as captain and player. I am immensely proud of the team. If we had managed our power plays better, we would have had better scores.
"However, there has been positive growth from last year."
During the tournament, South Africa beat Fiji and lost to New Zealand, Jamaica, England and Australia. In the position playoffs, South Africa beat Fiji 25-19 to claim fifth place.