There have been gaps before.
Between the 1st and 2nd movie there was about 6 years. 6th and 7th was about 2 years, the 10th and 11th was about 2 years.
Plus in 1 year, 2007, they put out both the 13th movie and the tv series, quite a bit for 1 year. So they may be taking a break or something.
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I've copied below the listings of when the movies came out for comparison.
1 The Land Before Time (1988)
2 The Land Before Time II: The Great Valley Adventure (1994)
3 The Land Before Time III: The Time of the Great Giving (1995)
4 The Land Before Time IV: Journey Through the Mists (1996)
5 The Land Before Time V: The Mysterious Island (1997)
6 The Land Before Time VI: The Secret of Saurus Rock (1998)
7 The Land Before Time VII: The Stone of Cold Fire (2000)
8 The Land Before Time VIII: The Big Freeze (2001)
9 The Land Before Time IX: Journey to Big Water (2002)
10 The Land Before Time X: The Great Longneck Migration (2003)
11 The Land Before Time XI: Invasion of the Tinysauruses (2005)
12 The Land Before Time XII: The Great Day of the Flyers (2006)
13 The Land Before Time XIII: The Wisdom of Friends (2007)
Tv Series - 2007