small angelfish

Hi all
In January I bought 5 angelfish to try to get a pair for breeding
They were all about 1" in size.
Four grew to about 4" but one is still 1" in size.
It is not ill, it behaves normally, feeds normally it just wont grow.
As Iam new to angelfish can anyone explain what went wrong.
All the best
Jim.

There are various factors that can affect growth rate

Firstly, out of any batch there will be some fish which are genetically weaker which don't grow as quickly.
Secondly, even if you choose 5 fish that are from the same batch and are identical in size, once in a tank on their own, one will become the dominant fish and get more than it's fair share of the food, and one fish will find it's self at the bottom of the pecking order and on the most impoverished rations.
Thirdly, some fish have the genetic potential to grow larger than others
Fourthly, some varieties are well known for being slower growing and/or not getting as large as other varieties...

zebra with two zebra genes, pearlscales, Blacks (and other very dark fish such as dark marbles), blues and albinos all can be slower growing.

Finally, intestinal parasites can really hold back growth. I might be tempted to treat it with some sort of wormer.