Do you know what type of Plec you have? Do you have a pic?
What kind of algae do you have? Do you have a pic?
You may have bought a "Common", but have something else entirely?
Some Plecs don't eat alot of algae, and not all algaes are considered edible by different types of fish. Some's dietary needs change as they get older, but it sounds like yours is still young, though some Plecs don't get very large.
It'll take a "very" large tank to support a medium sized or larger plec with enough algae to form the mainstay of its diet, plecs need "bulk", zuchinni, cucumber and assorted lettuces are good choices, some folks use beans-n-peas especially for young plecs.
Some Plecs need wood to eat to or they'll suffer ill effects, some are even prefer a meaty diet.
While the majority of Plecs are herbivores some Plecs are omnivorous and hence very opportunistic, so if other foods are readily available then they'll chow down on the easiest meal.
Some fish can be forced by need/hunger to learn to eat things that would usually be off-limits... Floating flake food and swimmimg upside down to get at it for example.
A picture could go a long way to
ID'ing your fish and helping with dietary information/needs.
And then we must accept the fact that fish are individuals and like us can be weirdos
...
If your gonna trade'em...
The absolute best algae eaters to be had
IMHO would be a small school (5 or more.) of Otocinclus, though even they don't eat all types of algae and will rapidly clean any tank and need supplemental foods.
There are alot of websites dedicated to Cats-n-Plecs, you may find a plec guru or two that can help you with your fish at one of these.
Goodluck
