One if condition works perfectly in laravel blade but another doesn't work

I have some problem in laravel blade i have simple table and first if condition works perfectly but second doesn't work maybe some one can say where is my mistake because condition almost similar to first if ;/

<h4 style="text-align: center;">Naujausi knygų išdavimai</h4>
<div class="pagrindinis">
    <table class="table table-bordered">
        <thead class="bg-warning">
            <th>Knygos pavadinimas</th>
            <th>Miestas</th>
            <th>Išdavimo data</th>
            <th>Grąžinimo terminas</th>
            <th>Vardas</th>
            <th>Pavardė</th>
            <th>Kliento nr.</th>
            <th>Veiksmai</th>
        </thead>
        <tbody>
            @foreach($paskolinimai as $p)
            <input type="hidden"
                value="">
            <tr>
                <td></td>
                <td></td>
                <td></td>
            @if($p->grazinimo_data != NULL)
                <td>
                    <strong style="color: green;">Knyga grąžinta!</strong>
                </td>
            @elseif($skirtumas > 0)
                <td>
                    Liko <strong style="color: crimson;"></strong> dienų.
                </td>
            @elseif($skirtumas < 0)
                <td>
                    <strong style="color: crimson;">Terminas praėjo!</strong>
                </td>
            @elseif($skirtumas = 0)
                <td>
                        <strong style="color: crimson;">Šiandien paskutinė grąžinimo diena!</strong>
                </td>
            @endif
                <td></td>
                <td></td>          
                <td></td>
            @if($p->grazinimo_data = null)  
                <td><a href=""
                            class="btn btn-primary">Grąžinimas</a> </td>
            @elseif($p->grazinimo_data != null)
                <td><a href="#" class="btn btn-success">Grąžinta</a> </td>
            @endif  
            @endforeach
            </tr>
        </tbody>
    </table>
</div>