It appears to be a Land Desktop issue. I created two plots (one with the Plot Upside Down option selected and the other without) and each one produced different results. When plotted one way, some images would disappear; when plotted the other way, the images that didn't plot the first time showed up, but others then didn't plot.
Long story short.... I cut/pasted both plots together, scanned them to one TIF and all is well (for now).
Thanks for all of your suggestions.