See this documentation link

Note that if you include an apex:outputField or an apex:inputField component in an apex:pageBlockSectionItem, these components do not display with their label or custom help text as they do when they are children of an apex:pageBlockSectionItem. Also note that apex:pageBlockSectionItem components cannot be rerendered; rerender the child components instead.

Not to worry though! Just do this:

<apex:outputLabel value="[Label value goes here]" for="[inputText's ID goes here]"/>
<apex:inputText value="{![your value goes here]}" id="[inputText's ID goes here]"/> 

Note however that you can only have two items inside a pageBlockSection item, so you may need to rework your page a little

EDIT: On that note, one way to get round the two item limit would be to use

<apex:outputPanel> 

...to bunch items together

See this documentation link

Note that if you include an <apex:outputField> or an <apex:inputField> component in an <apex:pageBlockSectionItem>, these components do not display with their label or custom help text as they do when they are children of an <apex:pageBlockSectionItem>. Also note that <apex:pageBlockSectionItem> components cannot be rerendered; rerender the child components instead.

Not to worry though! Just do this:

<apex:outputLabel value="[Label value goes here]" for="[inputText's ID goes here]"/>
<apex:inputText value="{![your value goes here]}" id="[inputText's ID goes here]"/> 

Note however that you can only have two items inside a pageBlockSection item, so you may need to rework your page a little

EDIT: On that note, one way to get round the two item limit would be to use

<apex:outputPanel> 

...to bunch items together