Yes, collector's items can lose value and they can be rather faddish, depending upon what you are talking about. For a time, anything Martha Stewart wrote about in "Living" went up in price as people clammored to buy it; still, period or items which become trendy can make a collector's item worth more for a time then they will settle back into a niche.
And, in a recession such as the one we are in now, it is a buyers market for some items, as it is hard to sell anything, so that, too can effect the value or current price of an item. But then, the stock market can go up and down too, even good solid companies..
Then there is scarcity which affects the item's worth.
That said, there are items which keep their relative value; but it is like any other investment, sometimes you have to wait for the right time to sell.