[This is part of a series of defectivity case studies.]

According to Sande (2015), only Amharic verb stems that contain a geminate can form a frequentative. Since not all imperfect aspect verbs have geminates, some lack frequentatives and speakers must resort to periphrasis. If I understand the data correctly, it appears that the frequentative is a /Ca-/ reduplicant template which docks to the immediate left of the first geminate; the C (consonant) slot takes its value from said geminate. For instance, for the perfect verb [ˈsäb.bärä] ‘he broke’, the frequentative is [sä.ˈbab.bärä] ‘he broke repeatedly’. But there is no corresponding frequentative for the imperfective verb [ˈjə.säb(ə)r] ‘he breaks’ since there is no geminate to dock the reduplicant against; Sande marks as ungrammatical *[jə.sä.ˈbab(ə)r] and presumably other options are out too.

(h/t: Heather Newell)

References

Sande, H. 2015. Amharic infixing reduplication: support for a stratal approach to morphophonology. Talk presented at NELS 46.