Normally the actuator for a lift rail is at the bass end.  This means
that the rail will always start lifting from that end onwards towards
the treble.  I have seen on some pianos that a lot of free play exists
in some of the bushings that guide the rail.  This can be the problem.

If the new hammers installed are really so much heavier than the old
ones, there will be a serious problem with the touch of the piano.
Start by weighing an old and a new hammer.  If the difference is more
than 10g, there will be more problems as mentioned.  Have them changed
again.

Test if the keys will lower with a weight of 50-60g all the way to
the bottom.  If they don't, you still have a serious problem with the
weight.  The piano might be ruined if you don't change the hammers for
something more suitable.

However, If no bushings in the rail have any play and if the piano does
play at 50-60g, then maybe you can try the following: If the rail can
be moved along the shank, further or closer to the hammer by means of
some nuts on the metal levers that connect it to the action, move it
towards the hammers as far as possible to give it a better leverage.

If this does not help, strengthen the rail so that it cannot bend down
by screwing on a piece of steel or brass.  Screw this in vertically at
the rear of the rail.  It will then definitely work.  Coming to think
of it, some rails do have a strip of metal on their side for exactly
this purpose.  Could yours be missing?

I repeat, weigh and check the action and new hammers again before
altering anything as mentioned above.  If it did work before and if
nothing else was changed, then something was done wrong and you should
not rectify it by introducing other cures and fixes but by undoing the
wrong deed first.

Regards,

Bernt W. Damm
Restorer of Automatic Musical Instruments,
Cape Town, South Africa