I have the same problem a bit in my dojo, usually the beginners. Often they do not realise what we are trying to achieve, and throwing them around is not an option. Usually I say that practise is practise of a particular technique and that aikido forms a whole, in which you do the technique appropriate to the situation, therefore outside randori the uke needs to attack in a certain way.
I think a misunderstanding with training is that the type of attack (e.g. shomen-uchi) is the important thing which decides the technique. However really it is the 'style' of attack which is important i.e. are they pushing forwards, pulling backwards (withdrawing), turning away from you, lifting their elbow etc.
Now, I have changed instruction to indicate WHY we do this particular technique rather than another. This helps uke to understand that if they move in a completly different way, practising this technique doesn't make any sense. For example; uchi sankyo I often say that it is from a strong grab where uke is putting a lot of strength into the grabbing arm - thus the elbow tends to comes out and the arm is fixed, allowing plenty of space for nage to slip underneath into a perfect sankyo.
I think it is very important to learn APPROPRIATE technique rather than the same technique from any attack (for example, it would be very dangerous to slip under ukes arm if his arm was in close to his body as Nage would probably end up in a choke hold).
Randori to me is the culmination of this practise where APPROPRIATE technique is used (rather than as many techniques as one can think of).