Toyota Corolla cranking but not starting

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Hi I have a Toyota Corolla from 2001 with the 1.4l vvti engine. I have a weird problem with my car and was hoping someone would have some answers.

I had my car in a workshop a few months ago to change the valve cover gasket on my engine. And after that visit to the mechanic my car started having this problem. So whenever I start my car, sometimes it can start just fine on the first try. But sometimes, and more frequently now my car won’t start on the first time. When I try to start the car it just cranks. The ignition is on and the Battery is working, but for some reason the car just cranks and won’t start. But then if I try again it starts, or starts after a couple tries. Then when the car is running, it runs well. We thought the problem was with the camshaft sensor or the crankshaft sensor so we changed both of them. But this problem still remains. So I’m wondering if anyone has had a similar problem with their car or knows what the problem could be?

I have also changed the Battery so it’s new. 

Any type of response is appreciated!