LookupCache is working weirdly, i.e it's retrieving cache sometimes and other times it's empty

[#1]

LookupCache policy configuration:

<LookupCache async="false" continueOnError="false" enabled="true" name="Lookup-Cache-2">
    <DisplayName>Lookup Cache-1</DisplayName>
    <Properties/>
    <CacheKey>
        <Prefix>pupulate</Prefix>
        <KeyFragment>cache</KeyFragment>
    </CacheKey>
    <Scope>Global</Scope>
    <AssignTo>myvariable1</AssignTo>
</LookupCache>

PopulateCache configuration:

<PopulateCache async="false" continueOnError="false" enabled="true" name="Populate-Cache-1">
    <DisplayName>Populate Cache-1</DisplayName>
    <Properties/>
    <CacheKey>
        <Prefix>pupulate</Prefix>
        <KeyFragment>cache</KeyFragment>
    </CacheKey>
    <Scope>Global</Scope>
    <ExpirySettings>
        <TimeoutInSec>3600</TimeoutInSec>
    </ExpirySettings>
    <Source>response.content</Source>
</PopulateCache>

What happens is: with LookupCache, the result is there for every 4th request, but for other it's not even present.
I am not getting what exactly I am missing. Please help.