Yellow leaves on clematis
 

Last year I bought 2 Clematis macropetalla "Pauline" one did very well
but the other never really got going. I dug it up and planted it in a
pot which seems to be helping (it has put out some new shoots) but the
leaves are rather yellow. The one planted in soil has much greener
leaves. I read somewhere that yellow leaves on clematis can indicate a
Mg deficiency and so have sprayed it with a dilute solution of Epsom
salts. i only did this yesterday so it's probably too early to tell
whether it will work or not, but would there be any other reason for
yellow leaves and poor growth?
Thanks