Were the plugs and leads changed because the car seemed sluggish?
Symptoms  sound like the car is not firing on one or more cylinders.
Sometimes the rubber shrouds round the plugs stops you pushing
them fully home, you can usually feel the contact seating on the plug.
A lot of accessory shops sell a cheap ignition tester, comprising
a yellow plastic body,  a neon bulb, and it goes on the plug, the
plug lead goes on the tester.
If the bulb flashes when you crank, it can be left in place if the
engine starts, it's flash indicates HT to the plug.
The plug may not fire if it's faulty, or bridged with a carbon
deposit, but you know that it should fire. Always assuming
that enough fuel is getting to the engine.
Many mobile call out mechanics use these, and for about
3UKPDS in most accessory shops a good investment.
Best to test in dim light as the flash isn't very bright.
Could still be the leads in the wrong order, you need the
firing sequence for the car, and then work out if it's right
from the distributor cap, some have the plug numbers
molded into the plastic.