Hi Reuben,
You do not say if this problem has developed slowly over a long period of time or has suddenly started, maybe after renewing the furflex?
Assuming the door is hung properly, , with no up/down play in the hinges. with even gaps and no fouling, you need to check if the latch on the door is working properly. The forked part on the door edge is designed to engage the stud on the striker plate. When it operates, it should click cleanly to two closed positions and then click back to starting position when you press the door button or use the inside handle. Check that it is moving freely, clicking to its two closed positions and then releasing smartly on the button. You also need to check that, when in the closed position, the latch is firmly secured, so after doing the two clicks, try to force the latch back again without using the button. If it yields, the latch mechanism is faulty or worn and is not safely holding the door. If the latch checks out OK, you have an adjustment problem.
If the door is not easily closing to its second secured position, something is blocking it. It may be newly fitted door seals or simply bad adjustment of the striker plate. Check that the mouth of forked part is cleanly engaging the stud on the striker plate, without fouling top or bottom. If it is, then the problem may simply be that the striker plate is adjusted too far in and isn't properly actuating the forked part to its second position. As the door closes, the stud engages the fork and moves it downwards through its two clicks. But it needs to do this fully before the door closure is stopped by trim and seals. Slacken the screws on the striker but don't remove them. Slide the striker plate out as far as it will go towards the door, lining the stud up with the fork. Tighten the screws again and check if the latch is working properly to both clicks, even if the door isn't flush with the body. If it is working OK and holding its position safely, then progressively move the striker plate inwards until you can only get one click, then back it off to the last good two-click position.
It is not unusual for new door seals to take a while to settle, so if that is the source of your problem, just keep adjusting the striker plate inwards, over time, until a good door fit is achieved.
The Magnette is well known for doors that close easily with a satisfying "clunk". Don't be satisfied with anything less. It should not be necessary to slam them hard.