upgrading units in civ3

[Karl Biedermann]

units can be upgraded - eg spearman to musketman . when my nation disposes of he strategic good needed - salpeter - and roads to that source and money enough for the upgrade.
but sometimes some of my cities can neverthelesse not upgrade .
WHY??????
I really do not udnerstand the mistery.
please explain.
thanks in advance
Karl-Eduard.Biedermann@t-online.de

[vmxa1]

It could be that the unit does ot have any movement left. It could be you do not have the gold needed. It could be that the city in question does not have a barracks. It could be that that town does not have the resources in its box.

That is it is not connected to the net. No road or harbor or it is blockaded.

So what is really needed is:

1) barracks in that town
2) enough gold
3) strategic resources in its box
4) unit having movement left
5) the required techs