Sejuani freeze stacks not stacking from allies

IDK if im stupid or missing something. Tooltip says allies add stacks to target. i see the "frost" animation going to the allies. at this point they're supposed to add stacks... but they dont. i'm always the only one who can add stacks and get it up to 4. at first i thought " ok, im just not counting right or we didn't hit him 4 times"... but after trying and trying the result is consistent. i find it impossible that in a teamfight i am only able to freeze one target cus apparently it's the only one the team has hit 4 times?? also, i've seen it clearly.. target running away with 3 stacks.. me and adc following him.. adc hits, hits hits, no stacks. i had to run up to him and hit him a 4th time to be able to freeze him. so what's the deal?