Why can't I get an overview polyline from my directions api request

I am trying to plot polylines that include waypoints on my map in Xamarin forms. Now, I'm sending the following request to the Google Directions API

https://maps.googleapis.com/maps/api/directions/json?mode=driving&transit_routing_preference=less_driving&origin=31 Clairwood Rd, Harare, Zimbabwe&destination=8 Joshua Nkomo St, Bulawayo, Zimbabwe&waypoints=via:No. 1 Railway Ave, R Mugabe Hwy, Kwekwe, Zimbabwe|via:55 Fifth St, Gweru, Zimbabwe|via:Shangani Rd, Harare, Zimbabwe|via:XVJX+62J, Chivhu, Zimbabwe&key=KEY

and I am getting the following response

{
    "geocoded_waypoints": [
        {
            "geocoder_status": "OK",
            "place_id": "EiEzMSBDbGFpcndvb2QgUmQsIEhhcmFyZSwgWmltYmFid2UiGhIYChQKEgmBVmGqRqUxGRFK3dLcshV1ExAf",
            "types": [
                "street_address"
            ]
        },
        {
            "geocoder_status": "OK",
            "partial_match": true,
            "place_id": "ChIJIf-owoJpNBkR4od3cGjZYCs",
            "types": [
                "establishment",
                "point_of_interest",
                "university"
            ]
        },
        {
            "geocoder_status": "OK",
            "place_id": "ChIJ5TwQo5mUNBkRNhE_iYH5nXU",
            "types": [
                "street_address"
            ]
        },
        {
            "geocoder_status": "OK",
            "place_id": "EiU4IEpvc2h1YSBOa29tbyBTdCwgQnVsYXdheW8sIFppbWJhYndlIhoSGAoUChIJcXSTo35UtR4RIh0xgqF3-AwQCA",
            "types": [
                "street_address"
            ]
        }
    ],
    "routes": [],
    "status": "ZERO_RESULTS"
}

But, when I send this one

https://maps.googleapis.com/maps/api/directions/json?origin=31 Clairwood Rd, Harare, Zimbabwe&destination=8 Joshua Nkomo St, Bulawayo, Zimbabwe&waypoints=via: Gweru Polytechnic, Gweru|via: M.S.U. Zvishavane Campus, Zvishavane&key=KEY

I get a favourable response.

Now, I'm not sure why I'm getting NO RESULTS response from the first request, could someone assist me as to why. I have researched and researched, I can't seem to find a suitable answer.

I guess I had not optimized my route. The correct way to send the request should be as follows:

https://maps.googleapis.com/maps/api/directions/json?mode=driving&transit_routing_preference=less_driving&origin=31 Clairwood Rd, Harare, Zimbabwe&destination=8 Joshua Nkomo St, Bulawayo, Zimbabwe&waypoints=optimize:true|No. 1 Railway Ave, R Mugabe Hwy, Kwekwe, Zimbabwe|55 Fifth St, Gweru, Zimbabwe|Shangani Rd, Harare, Zimbabwe|XVJX+62J, Chivhu, Zimbabwe&key=KEY
``