Have you changed the length of the hoses? Most pump have Hmax in meter (height) hose lenght can influence this giving more counter pressure thus less Qmax l/h. The lable must specify this capacity. If not, and nothing physical is restricting the flow, you might want to check the impeler and or it's axle for wear or dirt. Possible replaceble with a jbl spare part.
If that all aint it.. I guess just "old".. Burnd coil, if it has seperate windings one could be burned and than it looses capacity.. Unlikely, the magnet from the impeler, but red it is possible that these in circumstances kinda can depolarize.. But i have no idea how to check that without buying a spare one and see if it makes difference. If it doesn't it's a waste of money..