Problems with mercenary recruitment

I don't know whether this a RTW or and EB bug, but occasionally (especially in hellenistic regions) I am unable to recruit all the units.

What happens is the following:

Imagine there are 4 types of units

[][][][]

If I select any of the first 3, I get the unit, no fuss.

If I select the last one, it highlights, and disappears from the available queue but never arrives in my army (neither am I charged for it)

If I then exit the mercenary browser and reenter again, the unit card is back (but I am still unable to recruit it)

It seems that the order I select the units matters to some extent, so for example if I select them in this order

[1][2][4][3]

I might get the 4th one but not the 3rd one. If I then select the mercenary screen again, the 4th one is back and the 3rd one is now gone (and again I cannot recruit it)

Any ideas what this might be?

I'm having the same problem. As Carthage I took Syracuse and recruited 2 units of greek archer mercs. Several turns later I've decided I want more. They are available on the merc recruitment scroll, but they will not add to my army. I have plenty of money and slots available in my stack.