Why return null responsetext by api in t-SQL?

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SQL

T-SQL

I created a stored procedure in sql server to get json data with API.
This sp runs successfully manually at once. But I created sql agent job which constantly calls this sp. This job always works, it works successfully for about 3 hours, but then it returns null from the api. Why could such a thing happen? There is no problem with the api because when I call the same api with the c# windows service, I do not encounter such a problem.

Store Procedure(Get_Raw_Data)

DECLARE @ResponseText as nvarchar(max),@Token VARCHAR(100),@URL VARCHAR(100)

SET @Token='mytoken'
SET @URL ='myURL'

DECLARE @responseTextTable table(responseText varchar(max));

Declare @Object as Int;
--Declare @ResponseText as Varchar(max);
Declare @Body as varchar(max);

Exec sp_OACreate 'MSXML2.ServerXMLHTTP', @Object OUT;
EXEC  sp_OAMethod @Object, 'open', NULL, 'get',@URL, 'false'

Exec sp_OAMethod @Object, 'setRequestHeader', null, 'Content-Type', 'application/json'
Exec sp_OAMethod @Object, 'setRequestHeader', null, 'Authorization', @Token
Exec sp_OAMethod @Object, 'send', null, @body

insert into @responseTextTable (responseText)
Exec sp_OAMethod @Object, 'responseText'

select @ResponseText = responseText from @responseTextTable

IF ISJSON(@ResponseText)=1 BEGIN
    SELECT device_id, company_id, master_name, last_at,status,date,timestamp,tag.[key],tag.[value],tag.[type],JSON_VALUE(tag.[value],'$.name') tagName,JSON_VALUE(tag.[value],'$.value') AS tagValue
    FROM OPENJSON(@ResponseText)
    WITH (
            device_id NVARCHAR(max) '$.device_id',
            company_id NVARCHAR(max),
            master_name NVARCHAR(50)'$.name',
            last_at DATETIME2 '$.last_at',
            status NVARCHAR(max),
            date DATETIME2 '$.date',
            timestamp NVARCHAR(max),
            tags NVARCHAR(MAX) '$.tags' AS JSON
        )
    OUTER APPLY OPENJSON(tags) tag
END

SQL Agent Job Step

SET TEXTSIZE 2147483647
WHILE 1=1 BEGIN
	Exec Get_Raw_Data
	WAITFOR DELAY '00:00:45';
END

Please help me,why is this happening?
Thanks,

What I have tried:

changed varchar(max) to varchar(800).