SQL> create table test (col char(8));

Table created.

SQL> insert into test (col) values ('abc');

1 row created.

SQL> insert into test (col) values ('12ab   ');

1 row created.

SQL> select col, length(col) from test;

COL      LENGTH(COL)
-------- -----------
abc                8
12ab               8

SQL>

SQL> create table test (col varchar2(8));

Table created.

SQL> insert into test (col) values ('abc');

1 row created.

SQL> insert into test (col) values ('12ab   ');

1 row created.

SQL> select col, length(col) from test;

COL      LENGTH(COL)
-------- -----------
abc                3
12ab               7

SQL>

declare 
 mcount char(8);
 a number;
begin
	if :iss.tray_no  is null then
		g.dummy := g.cent('Tray No. Cannot be Blank',1,1);
		 raise form_trigger_failure;
               [B]this will never happen you just stopped the trigger in the previous line[/B]
		 go_item('tray_no');
	end if;
   select count(*) into mcount from iss
		where rtrim(lower(tray_no)) = lower(rtrim
[B]should it not be :iss.trayno without ''[/B]
(':iss.tray_no')) and
		  sl_no <> :iss.sl_no;
      if mcount > 0 then
  		g.dummy := g.cent('Tray No. already exists',1,1);
		raise form_trigger_failure;
	end if;	

a :=	length(':iss.tray_no');
[B]a will always be > 8, you probably meant length(:iss.tray_no)[/b]
if a != 8 then
	raise form_trigger_failure;
end if;
[B]you probably meant without :, also if a <> 8 you never get here[/B]
go_item(':iss.qty');
end;

[b] and [/b]