Theoretically your procedure is right, however there is margin for errors, in particular: how old is the stock of HCl that you used to prepare 0.1N solution. I ask because HCl is very volatile, in fact max concentration is 37.4%, that is true when the bottle is new and well kept in a dark cool place. If the bottle was left open for a while you might have a different concentration which could explain why you have a diluted HCl solution. One way to find out which could be to check the pH, attached there is a file that tells you the pH of HCl solution at defined concentrations. Another source of error could be the volume measurements, how do you do your titration? And Finally, do you know for sure that your NaOH is 0.1N? 

My bet would be that the problem is rather with your NaOH solution. It loves to absorb CO2 and hence to form the familiar carbonate buffer system. It is much easier for NaOH to go bad than HCl.

Hi how about switching sodium hydroxide solution with sodium carbonate solution ? You can dry sodium carbonate before solution preparation and normally more stable compare to sodium hydroxide. As stated by Mr. Domenico, how do you know your sodium hydroxide solution is truly 0.1N ? Do you standardized it with other solution e.g. KHP solution before you used it for titration ? 

Thank you all for your answers

NaOH was obtained recently and was prepared at the moment before titration. I calculated 0.2 g of NaOH to prepare 50 ml of NaOH 0.1 N according to the following formula:

 N = eq / L ---> where: eq = g / (M.W. / Valency)

 g = N x L x (M.W. / Valency)

Finally,

  g = 0.1 N x 0.05 L x ( 40 g/mol / 1 ) ---> g = 0.2

On the other hand, HCl is 37% (amber bottle) and it is kept in a dark cool place. I'll see its expire date and check pH of HCl 0.1 N (that must be 1).

Question: What do you think about boil water before dilute NaOH? 

NaOH in pallets may be covered on the surface of some sodium carbonate and this could be one of the source of strange results.

As previously stated sodium hydroxide is not stable since may fix the environmental CO2. It could be better to use a more stable reference as sodium carbonate. 

I would also suggest to use a different hindicator than phenolphtalein because its pH range is between 8 and 9 pH units and the equivalence point is reached at pH 7 for this reaction.  So when you see the color change the equvalence point is aready passed.

May be methyl red or red bromocresole are more suitable since their pH ranges are more near to the equivalence point.

Hi I don't think boiling water can solve your current problem because you are having a higher concentration of sodium hydroxide solution than expected. 

Based on your calculation, either your HCl solution is lower than expected or NaOH solution's concentration is higher than expected. HCl usually much more stable than other acid e.g. Nitric acid, thus I less concern the acid's original concentration. 

Do you record the weight of sodium hydroxide you used for calculation ? Besides that, I strongly recommend you standardize your sodium hydroxide solution with potassium hydroxide phthalate solution before you use it for titration.

If the sodium hydroxide solution prepared is having a correct concentration, maybe you need to check your acid solution. Either you use a new bottle of acid or you titrate it with a higher concentration of alkaline solution. Did you check your apparatus for any possible contamination ?  

As for indicator, phenolphthalein should work fine at this point as it's a strong acid-base reaction. But if everything mentioned above does not solve your problem, maybe you can try use other indicator e.g. thymol blue (green at neutral, yellow at acidic, blue at alkaline condition) or do a potentiometric titration which is more accurate than using indicators.

Hope you solve your problem soon.