My best guess is because the basic nature of sildenafil citrate, chitosan would repulse sildenafil citrate since both of them are positively charged, and thus preventing the entrapment of sildenafil citrate. Rather than only using chitosan, I would like to suggest using negatively charged polysaccharides like alginate or LMP (low-methoxyl pectin). It can be used alone with ionic gelation using Ca2+ or together with chitosan. There are many research that combine chitosan with negatively charged polysaccharides.