I'm thrilled to finally read about Rendaku (連濁) in my learning journey of Japanese, and to realise that ときどき is a simple example of this phenomenon!!
However, "少々" in 少々 お待ちください still remains as "しょうしょう" instead of "しょうじょう" (with tenten on the second "少")
a) Is there any specific reason why Rendaku is not applied in this case?
b) I read that the exceptions to Rendaku are sometimes due to unpredictability and also due to sound-words.
{{pad}}There are many exceptions, but the general rule is that the second component of an on-yomi compound is not voiced. This includes on-yomi reduplicative words like 少々【しょうしょう】, 多々【たた】, 点々【てんてん】, 嬉々【きき】, 着々【ちゃくちゃく】, 淡々【たんたん】, 個々【ここ】 and 早々【そうそう】. There are exceptions like 種々【しゅじゅ】 and 方々【ほうぼう】. Ultimately, you have to learn them one by one.
See: Rules or criteria for 連濁: Voiced or unvoiced syllables in compound words
Most Sino-Japanese words tends to resist rendaku. 保安 (ほあん; safety-keeping) + 検査 (けんさ; inspection) = 保安検査 (ほあんけんさ; safety inspection), not ほあんげんさ. (But some words accept rendaku. 株式 (かぶしき; stock) + 会社 (かいしゃ; company) = 株式会社 (かぶしきがいしゃ; roughly corresponds to “business corporation”).)
