A stop with a liquid after a short vowel need not make the preceding syllable long by position. A syllable containing a short vowel before a stop and a liquid is common (either short or long). When short, such syllables are said to have weak position.
Thus, in δάκρυ, πατρός, ὅπλον, τέκνον, τί δρᾷ the first syllable is either long or short as the verse requires. In Homer the syllable before a stop with a liquid is usually long; in Attic it is usually short.
a. The stop and the liquid making weak position must stand in the same word or in the same part of a compound. Thus, in ἐκ-λύ_ω
b. β, γ, δ before μ, or ν, and usually before λ, make the preceding syllable long by position. Thus, ἁγνός (¯˘)
N.—‘Common’ quantity has been explained as due to a difference in syllabic division. Thus, in τέ_κνον, the first syllable is closed (τέκ-νον); while in τεκνον the first syllable is open (τέ-κνον). Cp. cross141.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].