API Lead response returns empty result array half of the time

[Anonymous]

I am using Marketo REST API to get lead information, and I apply a filter of _mkto_trk cookie value.

My request looks like: https://078-whz-188.mktorest.com/rest/v1/leads.json?filterType=cookie&filterValues={cookie_value}&access_token={api_token}

Half of the time it works great, but then many times I get an empty result array back. What can be the reason for that? I even waited for an hour after new cookie was created, but still I see empty result array. Here is a screenshot: