In Tehilim ch. 25 it goes through the entire alphabet and skips the letter "Bais" "Vav" & "Kof"; why so?
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R. Yosef Chiyun (linked in @jake's comment here) says that these three letters spell out בוק, meaning "emptiness" (as in הבוק תבוק (Is. 24:3), "thoroughly emptied"). So the omission of these letters implies that "one who says [this chapter of Tehillim] will not see בוק."
Pri Eitz Chayim (Shaar Nefilas Apayim 2) explains that בוק also suggests "a flame" (as in אבוקה, "torch"), and that conversely there are three letters that are repeated in this chapter - א, פ, ר, spelling out אפר ("ashes") and its anagram פאר ("glory"). So, he says, מאחר שהוא מכין עצמו ונפשו, להיות כמת ואפר תחת רגלי הצדיקים, זוכה להיות פאר וניצל מגיהנם ונכנס לג"ע - "once a person [when saying this chapter as part of Tachanun, as in Nusach Sephardi and Nusach Ari] prepares himself and his soul to be considered as though dead, and 'ashes' under the feet of the tzaddikim, he then merits 'glory,' and is saved from [the 'flames' of] Gehinnom and enters Gan Eden."
[The caption to this chapter in the Ohel Yosef Yitzchak (Lubavitch) Tehillim also associates the missing three letters with Gehinnom, though in a different way: בוק numerically equals גיהנם. (The printed text there seems to be corrupt, since it says that this gematria requires עם הכולל - adding one for the word as a whole - when in fact that's not necessary.) I'm not certain of the provenance of these captions, although I believe I've heard that they are from the Shaloh.]
Radak (ad loc.) says the answer to your question (and why resh appears twice) is "not known, really" (though he first quotes ibn Ezra (though not by name) who suggests that the missing bes and vav are held by "בך" (verse 2) and "ולמדני" (verse 5) respectively).