was using Remington 55 GR. I will try a couple different magazines and see if that works!
That ammo should work fine.
Do you know if there are any rubber buffers installed between the recoil spring and the receiver? If so, this could cause short stroking of the action that could interfere with cartridge pickup from the mag.
As you have not given any detail about how your Mini fails to feed one has to guess at all the possibiities: How far from closed is the bolt when your Mini jams?
If the action has too much friction so that the bolt does not have enough force to close, then a new recoil spring may help - don't buy "extra power" recoil springs at that can lead to short stroking or close the action so quickly that the mag spring cannot keep up.
Get hold of an owner's manual for your vintage Mini and make sure that no parts are misassembled - on the current models it is possible to assemble the recoil spring guide rod upside down and bind up the action.
Make sure al mating surfaces of your Mini have some lube on them. Any lube is better than none. I like to mix a little grease and CLP - either or both of these are better than dry. Don't forget to oil the extractor pivot and spring plunger - have you checked to see that the extractor snaps all the way over the cartridge rim - do this with the bolt out of the rifle or with live ammo at the range.