Can’t complete any sort of scan

I am trying to scan some websites I have on my server for malware due to suspicious acitivty. I have tried every plugin and option possible: Sucuri, GOTMLS, Wordfence etc and all scans fail after a couple of seconds and do not continue. The sites are rather large but it shouldn't mean the scans fail as such.

Can anybody shed some light on what this could be and potential fixes? Thanks in advance