Model based property failed after few tests

Question

I am developing a property based test generator in Erlang. I have developed all the properties successfully, but only model based properties are failed after few tests.

Here is my code:

prop_delete_model() ->
    ?FORALL({K, T}, {atom_key(), bst(atom_key(), int_value())},
        case model_delete(K, model(T)) of
            [] when T =:= leaf -> T =:= leaf;  % Handle the case where the tree is empty
            ModelResult -> equals(delete(K, T), ModelResult)
        end).

-spec model_delete(Key, [{Key, Value}]) -> [{Key, Value}].
model_delete(_, []) -> [];
model_delete(K, [{K, _} | Rest]) -> model_delete(K, Rest);
model_delete(K, [Head | Rest]) -> [Head | model_delete(K, Rest)].

prop_union_model() ->
    ?FORALL({T1, T2}, {bst(atom_key(), int_value()), bst(atom_key(), int_value())},
        equals(union(T1, T2), model_union(model(T1), model(T2)))).

-spec model_union([{Key, Value}], [{Key, Value}]) -> [{Key, Value}].
model_union([], T2) -> T2;
model_union(T1, []) -> T1;
model_union([{K, V1} | Rest1], T2) ->
    case find(K, T2) of
        {found, _} -> [{K, V1} | model_union(Rest1, delete(K, T2))];
        nothing -> [{K, V1} | model_union(Rest1, T2)]
    end.

And this is my delete and union function's code:

delete (_K, leaf) -> leaf;
delete (K, {branch, L, Key, V, R}) ->
  if K < Key    -> {branch, delete(K, L), Key, V, R};
     K > Key    -> {branch, L, Key, V, delete(K, R)};
     K =:= Key  -> join(L, R)
  end.

union (leaf, R) -> R;
union (L, leaf) -> L;
union ({branch, L, K, V, R}, T) ->
  {branch, union(L, below(K, T)), K, V, union(R, above(K, T))}.

After checking the tests it is failed. I could not find the error. I am using the commands eqc:quickcheck(test_bst:prop_delete_model()). and eqc:quickcheck(test_bst:prop_union_model()). to check the result and it is showing the result like this:

prop_delete_model: .......Failed! After 7 tests.
{g,{branch,leaf,c,-1,leaf}}
   {branch, leaf, c, -1, leaf} /= [{c, -1}]
Shrinking .x..(3 times)
{a,{branch,leaf,a,0,leaf}}
   leaf /= []
prop_union_model: Failed! After 1 tests.
{leaf,leaf}
   leaf /= []

How can I solve this?