It may be due to various reasons. One being traffic between you & them. Another being them using their bandwidth for others (which varies) or for another network. The net & gnutella are dynamic.

What I personally do is, if it's a recently chosen downld, I will stop it, then re-choose it from the search results. For some reason this seems to refresh it. Another is, if at all possible, I'll browse the host I'm downlding from. And I'll also try direct connecting with them. If you can browse them, you can obtain the ip no. & use that to direct connect. If it's a large file or/& they have more files you're after, you can make a note of this no. for the future. So next time you connect using LW, you can attempt to direct connect to them (that is after you've left LW on it's own to try to connect to whatever it can of your incomplete files & it's settled down.)

Both browsing host & direct connecting can both have the same effect as re-searching the topic again (sometimes even better than reselecting from search results since you may find extra hosts. eg: Those hosts may have connected onto other links to hosts with similar files in the meantime.)