These often arise from a combination of (i) inadequate loop length, (ii) poor choice and diameter of loop material, (iii) losses in the variable capacitor and (iv) high resistance connections, especially between the loop and the variable capacitor. In addition compromised parts can mean that the loop handles only a fraction of the legal power limit. Add the loop's losses with the reduced signal due to a restricted output power and suffer the consequences. You may still do well on efficient digital modes like WSPR but you will likely be the weakest station in the group if on an SSB net.
It doesn't have to be like that. With time, money and effort you can build a heavier, more efficient loop that can take the legal limit. Your signal will then be quite strong. In fact under certain circumstances you might do better than some with full-sized antennas. That's even true on lower bands like 80 and even 160 metres.
Building a big thick and efficient HF magnetic loop is more than your average casual weekend project. But the results will be worth it. Here's a few resources from those who have succeeded in this quest.
VK4AMZ magnetic loop for 160, 80 & 40m
W7YV magnetic loop for 160, 80 & 40m
VK5SFA 160m magnetic loop design
VK5SFA's 160m magnetic loop videos
AI6TK 160 and 80m magnetic loop (YouTube video)
Even if you don't attempt one, these links should at least give you a renewed appreciation of what magnetic loops can do when attention is paid to minimising losses from all sources.
PS: I have written five books on amateur radio topics. They are available in electronic and paperback form (most countries). Ebooks are under $US 5 each. Find our more here or follow VK3YE Radio Books on Facebook .