What Causes Cervix To Be Tight?

Hi .I am 29yrs i have had an hsg 2yrs back and blocked tubes were diagnosed, which i got it opened thru laproscopy. Now i want to concieve but is not able to. So my specialist told me to go for hsg, which was unsuccessful twice, as they say that my cervix is too tight. i want to know, what causes the cervix to be tight and can it be a barrier in conception, where as my period are regular and normal.