Cupler trouble

[Hwood42807]

New to trains, my son reviewed a Santa fe ho scale for Christmas.  He also got a few 'new to him' additional cars.  All have had cupler replaced however, they still disconnect from other cars sometimes.  A couple of the cars that came with the Santa Fe also uncuple sometimes.  Any ideas???

[jbrock27]

Coupler

What kinds of couplers are these?  Are they knuckle couplers or horn hook style?  And w/o the benefit of further info from you, my first guess is the coupler heights are not uniform.  If these are knuckle couplers, do they have the tiny metal spring on the knuckle or a thin plastic sliver instead?