mixing problems with new bath/shower taps

[crompton]

I've just had new bath taps with a shower hose fitted. Tenants report they can only get fully hot or cold, nothing between.
It shouldn't be a boiler issue as the only thing changed is the taps. The old ones were just old and ugly and stiff changing from bath to shower, but the mixing worked fine. Is it a matter of tweaking something in the taps?
Any suggestions?
Thanks.

 

[crompton]

Looking around, people suggest that the new install might be with narrower tails: I'll have to check with the plumber but does that sound possible?
Or the lack of an aerator?
Either of these sound plausible? Or any other suggestions?
Ta