Guernsey coach Jordan Reynolds says a number of his players let him down as they lost 22-21 at Sidcup.
Reynolds says his side's loss, and last week's defeat by Old Elthamians were down to his players' "stupidity".
"In the last two weeks it's not that the teams were better than us, we were just losing the match by our own stupidity," he told BBC Guernsey.
"It's only a couple of players that are letting the rest of the guys down," he added after the London One South match.
Guernsey dropped to fifth in the table, while Sidcup are third with a perfect record of four wins from four.
"After the game I had a few words and spoke a few home truths to some people because you just can't afford to do that kind of stuff come game-time," Reynolds added.