I own a boxer bitch who is 18 months old.  She had her first season at 10 months old.  She is now 18 months and has not had her second season.  Can anyone give me advice, if this is common or if I should seek advice.

Bitches can vary in their heat (season) cycles.
I owned one Boxer bitch who cycled once every 12-13 months.
Another of my bitches can into season at 9 months of age and cycles every 7-9 months
as does her daughter.

To be honest you'll need to wait until her second season to see how she may possibly cycle
in the future. If she does not have a season say 14 months after her first season so that would make her
24 months old then take her to your vet, unless she's gives any cause for concern, in any health matter
before then.

A book that you may find helpful is the book of the bitch, although it's known as the 'breeder's bible'
it is also a very helpful reference book for any 'bitch' owner regardless of whether they want to breed or not.

hiya
i have 2 boxers, Lucie had her first season at 7mths old, her second at 16mths and has just finished her third at 23mths.
so you can see she's varies her timings in seasons, bitches seem to get you all worked up and in the end,there's nothing to be worried about :)
she'll come in season when she's ready, don't worry
but, there's always a but, if she hasn't come in season by the time she's 2, i'd advise you to see the vet
or if you're that concerned,take her now, just for peace of mind
i'm sure there's nothing to worry about,she's still a baby and hormones are running riot :)
take care

boxacrazy
beat me to it :)