When the T-Rex turned up it was difficult to make it stand up, and within a few weeks it became impossible to get the model to stand up at all (it simply fell over).
The reason this happens are i) the base (the feet) are not flat; ii) the feet are very small to act as bases for such a large and tall toy (although I can't blame Schleich for the anatomy of a T-Rex, so I'll let them off and just say that it isn't really a suitable animal to be modelling); and iii) if you look at the picture, you can see that the head which is to the left of the feet is much larger than the tail to the right of the feet, as such it's centre of gravity is displaced forward (meaning it falls over).
Perhaps they should just re-label the toy "Dead Tyrannosaurus Rex", then it can lye happily on it's side.
That said, mine may just have been a one off `dud' (but given the issue with the centre of gravity I suspect it is a wider issue).