Report Generation Time Increases

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SQL-Server

SSRS

hi i created a report and when i start running it for the first time it is fast it wqas fine for couple of days but later on when i run the same report it took like 15 min and later on the time went up , after that i created indexes on table and the genaration time went down to less than 2 min and later on the time went up , i tested it for same amount of data ,and i am the only one who run it , why is this happening is there any strong reason , i am the only one who access the server ,but still i dont get it , can any one help me out thanks