VLOOKUP Returning random Errors

Hello. Just joined here becuase I have a strange problem with VLOOKUP.

I have workbook created that looks at data from two separate data logs. Each data log records the time and date, once per second, along with the series data needed. The data logs both run at the same time, but on different devices, so I have to coordinate the data together to look at it all at once..

I am using VLOOKUP to use the timestamp from one datalog to coordinate the data from the second datalog. There are 2800 time stamps in my current file. Out of those, there are about a dozen places where the VLOOKUP function returns an error. When I look at the time stamps, they match exactly, just like all the other ones that return correct data. I have the ISERROR function in use also, to return a null cell in the event no data is available (this is to prevent problems at the end of the time series data, rather than showing a bunch of cells that say N/A)

(I've also tried removing the ISERROR part and then the cells in question just return N/A anyway)

Has anyone ever seen this type of problem?

Here is the formula I am using in the cells "=IF(ISERROR(VLOOKUP(A334,PQA!AR$10:AS$4000,2,FALSE)),"",VLOOKUP(A334,PQA!AR$10:AS$4000,2,FALSE))"