Dull, stupid.
Bashful, sheepish.
Physically unable to speak; dumb, mute.
Reluctant to speak up; reserved, reticent.
Having tongue-tie or ankyloglossia (“a congenital oral anomaly in which the lingual frenulum (a membrane connecting the underside of the tongue to the floor of the mouth) is unusually short and thick, decreasing mobility of the tongue and affecting eating, speech, etc.”).
Prevented from or unable to express oneself clearly or fluently, or freely; at a loss for words, speechless.