More than likely here you have placed the readmore in a place in your article that leaves html elements unclosed when only the intro is rendered. i.e. (in psuedo code)
This would be incorrect and break the columns:<div>
<p>intro para</p>
...READMORE...
</div>
<p>full para</p>
This would be correct and the columns would be fine:<div>
<p>intro para</p>
</div>
...READMORE...
<p>full para</p>
Regards, Mark.