need help

hey my car starts in the morning all the time but i could be driving around all day and stop to run into the shop for something and when i get back out the car wont start this has been happen for a few weeks now when i turn the key its like its trying to start but cant the starter motor is not clicking or anything i brought it to the garage today and their puzzled they checked the battery alternator connections everything and they z its all good that they would have to c the car when it cant start to get a better idea what it is i got them to change the spark plugs while it was in their as well has anyone any idea what could be wrong really sick of it now (rover 416)

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