Soft pickles are the most common complaint from people new to fermentation. The good news: the cause is almost always one of three things, all easy to fix.
Temperature too high. Above 20°C, fermentation is fast and uncontrolled. Find a cooler spot — 15 to 18°C is ideal. Salt too low. Below 3%, the wrong bacteria thrive. Stay at 3.5%. Cucumbers not fresh enough. Two-day-old cucumbers will never be as crisp as ones fermented the day they were picked.
Adding horseradish leaf, oak leaf, or grape leaf also helps — the tannins firm the texture. Traditional recipes include these for exactly this reason.