Hi st peter.
It could be any of three things.
The tube
The starter
A weak ballast
Consider buying an led direct replacement tube for the size of fitting you have. Eg. 4ft, 5ft, 6ft....
These tubes come with a starter module. It is only a built in link wire and will never burn out like a starter can.
They are relatively cheap. From around 6 0r 7 pounds rising to about £12 for a name brand like Philips
They don't fade over time like fluorescent tubes. And will last a lot longer WITH savings on your electricity bill.