moving average filter - unkown signal issue

The moving average filter consists of 3 components:

Counter - to increment the ram address

Ram - to store 4 data samples

average logic - computes the average and the result is latched

These three components I have tested using a testbench for each component an the results were as expected.

However when I included a top level design file that instantiates these three components, the internal signal takes the value of unknown when it should be summing that data at the input. (The signal initially is cleared by means of an asynchronous reset pulse)

p_average_logic : process(i_clk, i_rst)
		
	begin
		if(i_rst = '0') then
			o_dout <= (others => '0');
			dout <= (others => '0');
		elsif(rising_edge(i_clk)) then
			if(i_en = '1' and i_ce = '1') then
				dout <= dout + ("00" & i_din);
			elsif(i_en = '1' and i_updt = '1') then
				o_dout <= dout(13 downto 2);
			else dout <= (others => '0');
			end if;
		end if;
	end process p_average_logic;

Attached is the result of the simulation of the top level design. Between 60 and 90 ns of simulation time the signal goes unkown, in that step it should be executing the following code. (taken from the code shown above)

elsif(i_en = '1' and i_updt = '1') then
			o_dout <= dout(13 downto 2);

This occurs with a warning from the simulator : # ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).# Time: 60 ns Iteration: 1 Instance: /testbench/U2/U3

What could give rise to such an event? Especially since the component has already been tested individually?