I don't know how much money you want to spend on ammo (like you haven't spent
enough already, right), but I would try to pick up a box of each of these to try:
(all factory ammo)

Federal 140gr. Classic
Federal 175gr. Classic
Remington 140gr. Core-Lokt

It does sound a little like your rifle may prefer the heavier bullet weights (175gr.).
I suspect that was what you tried from your description of the cartridges. The above three represent most of that which is readily available for the 7x57 Mauser. Winchester
also loads a factory round, 145gr. bullet, but I don't see that around here much, and I think PMC has a couple loads. I think Federal also loads a Premium 140gr.
That's about it for factory ammo.
Great cartridge, the 7x57. Darn shame there isn't more interest in it, and more variety
in ammo. Mild recoil, generally very accurate, good penetration, and terrific deer cartridge. Be patient, the 7mm Mauser is worth it.