This question came as I analyzed the origin of the english words "happy" and "happen" and after my research i found the reconstructed proto-germanic root "hampijaną". However i found that this root only appears in the North Germanic languages and was probably adopted into english from Old Norse. Is it common for some roots to be adopted only in one of the germanic branches?How come it completely disappeared in western germanic languages such as German,Dutch,Low Saxon etc.?
Words die out and become replaced by neologisms at any time. When it happens shortly after a major fork into branches we see only one branch retaining the original word.
It is not really uncommon to find attestations of a certain word only in some sub-branch of a language family, both North Germanic and West Germanic have words that are absent from the other branch.
EDIT: For the definition of a branch, shared innovations (neologisms, sound shifts, grammatical innovations) are necessary. Therefore we see new words (of whatever origin, e.g., composition, loan words, new inventions) in each of the branches as well.
“Happy” and “happen” were both formed within English from the noun “hap” (good fortune). The OED (Oxford English Dictionary, on-line version) indicates the etymology of “hap” as follows:
Etymology: Probably < early Scandinavian (compare Old Icelandic happ (neuter) chance, good luck, success (Icelandic happ), and Norwegian happ, Swedish regional happ, both masculine), cognate with Norwegian heppa to happen, Old Danish hap (adjective) lucky, Swedish hampa (reflexive) to happen (by chance), (regional) happa, habba to happen, succeed, Danish happe, and further with Old English gehæp (adjective) suitable, convenient, gehæplic (adjective) convenient, orderly, probably < the same Indo-European base as Early Irish cob victory (rare) and Old Church Slavonic kobĭ destiny.
If the Celtic and Slavic parallels are correct, it follows that this root is not restricted to North Germanic, but has to be ascribed to proto-Indo-European. Obviously, it has not been preserved in all branches of IE, or even all branches of Germanic. But this is the way that languages work.