Structural steel welding has to be strong because you don't want the welds to fail and create structural danger for occupants and people nearby. Unfortunately, it's all too common for amateur welders to try to weld something together only for the weld to become unstable. Rather than trying to solder or weld something yourself, you need to hire a steel welder for the job, even if it seemed insignificant. All it takes is a tiny problem to make a weld go bad.
The Welding Material Could Be Contaminated
Welding material can become contaminated with flux particles or gases when the weld pool hasn't hardened yet. Any type of contamination of the welding material can make it weak and prevent it from setting properly. The intrusion of gases can weaken the integrity of the weld; the welder would have to start over and ensure that there was no moisture or other contaminants on the metal edges, which need to be clean and dry for the welding to succeed. Flux contamination simply prevents the weld from setting.
Something Was Not Set Up Correctly
Welds that are too thin, bad angling of the welding tool, incorrect setup for the welding process and use of the wrong tool can lead to weak, cracking welds. It can be tricky to get a weld right, which is why professional welders undergo extensive training; even they know they need to be very careful as they set up equipment and proceed with the welding. Someone onsite who knows how to handle a welding electrode may not take all the correct precautions or steps before beginning to weld.
The Welding Equipment Could Be Breaking Down
Even the best welder sometimes has to contend with older equipment that needs repair. It's possible that the equipment used chose that time to start breaking down. If the welder can't find intrusions or contamination and is certain that the right welding type and equipment were used, a look at whether the equipment needs repair is in order. This is a simple issue to fix; either the equipment is repaired, or the welder brings in different equipment and redoes the weld.
Don't let a small welding job fool you into thinking you can pick up a welding torch and just join something yourself. It's worth the money to bring in a professional welder for structural steel projects. You want that expertise for anything that needs to be welded together.