If these are electronic transformers, then you will need trailing edge (reverse phase) dimmers. The dimmer should say something like: "for use with electronic transformers"
If it is a magnetic transformer, then you will need a magnetic low voltage dimmer. Again, it will spell out what it is. The transformer will also tell you what type of dimmer you ought to be using. I'm not sure what kind of dimmers you bought, but if they match up correctly with the transformers, then there shouldn't be a problem. If you feel quality may be suspect, buy Lutron dimmers. They've always done me right. They are not cheap, however. A Diva magnetic low voltage dimmer rated at 600 watts will run about $80.
The other thing it could be is the load. Are there too many lamps on the circuit for the dimmer/transformer to handle? If the dimmers are ganged together in a box, you MUST derate their capacity. The fins that you break will let you know by how much.