It does make a difference. I used liquid lead which is tiny heavy balls that are used in model yacht building to weight down the hulls.
Plastacine can also work.
If they are Hornby Freightliner flats, there is room in both the very centre "Box" structure in the middle, and on little indents underneath the bogies themselves which may only seem like little areas, but they all add up. If they still give issues there is a mode expensive fix. Change then 10.5mm wheels to metal ones instead and for ultimate rolling, add brass bearings (Though caution needed if adding brass bearings as the bogie frames can be brittle so take it easy with the drill).
[If one ends up with spare plastic 10.5mm wheels, send them my way!!!
]
I did get one Hornby Freightliner waggon to run beautifully as a flat without containers when I added weight and tried heavy Romford wheels on it (Sadly these wheels are no longer available). The wagon had just enough weight to it and rode perfectly.
I eventually sold the fleet of Freightliner wagons on when I went into 7mm narrow gauge instead. Some sold at a toy and train fairs and others sold via this site. Kept my car on the road through the difficult times.
In the past I bought a rake of Bachmanns lovely bogie hopper wagons, and these had the complete opposite problem. They were just WAY TOO HEAVY! (Lovely build quality with the cast metal underframes but....!)
Eventually sold them on too. It took two central motored all wheel drive Bachmann diesels to pull just ten of them! If I used the old Lima diesels, it would have taken three of them to pull that, and the prototype diesels pulled around 40 of them using just one diesel!
