taxonomy_get_tree return only few terms
Try this on your code:  taxonomy_get_tree(1,0,NULL,TRUE)

Syntax:

taxonomy_get_tree($vid, $parent = 0, $max_depth = NULL, $load_entities = FALSE)



  Parameters
  
  $vid: Which vocabulary to generate the tree for.
  
  $parent: The term ID under which to generate the tree. If 0, generate
  the tree for the entire vocabulary.
  
  $max_depth: The number of levels of the tree to return. Leave NULL to
  return all levels.
  
  $load_entities: If TRUE, a full entity load will occur on the term
  objects. Otherwise they are partial objects queried directly from the
  {taxonomy_term_data} table to save execution time and memory
  consumption when listing large numbers of terms. Defaults to FALSE.