DIYbanter - how to reduce friction in dog-harness-belt virtually *impossible* to lengthen?

There's at least (I think) several types of around-the-chest
dog around-the-chest-and-neck harnesss.

The way mine (well, my dog's) works, for adjusting the
fit, is the belt-like material feeds through a little
plastic buckle or whatever it's called, doubling back
on itself, via a *very* tight fit.

Even when new, it's a bear to force a bit of the belt
up through it a bit, forming a wee loop barely large enough
for a tong (?) of "fine" needle-nose pliers into it,
so that with that you can pull a bit of the belt through it,
and pull down the other hole's part -- I guess you have
to see it to understand it.

ANYWAY -- once the harness gets a bit used, pushing and pulling
the belt through that buckle-thing becomes, well, virtually
*impossible*. Apparently wedged in, stuck tight.

QUESTION: what can you do to make it slide easier? Probably
not eg 3-in-1 ie petroleum-based oil, I suppose.

Ideas?

For those who have this same kind of harness for their dogs,
how do YOU adjust the size?

(Reason I need adjust the size: small dog (bichon), one month
or sometimes more between haircuts, outward-thickness of "coat"
is *so* much less after a haircut (gets a "puppy cut") that
if not shortened he can get his front legs out of the
around-the-chest part of the harness.

I've decided to buy another harness, and adjust it for apres-haircut,
and the other for just before haircut. But the current one
is just *impossible* to adjust.

Thus my problem.)

THANKS SO MUCH!

David