adding labels using PUT method on a issue doesnt work when called on quick succession

[Bala]

path: <domain>/rest/api/latest/issue/<issue-id>
json body: "{"update":{"labels":[{"add":"some-label"}]}}"

the api call works fine - for individual calls

api returns success status on multiple calls but it doesnt perform the task, like in above ex: add a label , when called in quick succession.
when there is time lag between calls - it works though !