If "SeriesNum" is a string, then a string value of "9" would be considered greater than a string value of "10".  When you reach "10", your DMax would still return "9" as the max, and you would keep getting "10" as the next value.  That's my guess for what is happening.  

String comparisons are done by comparing each character of two strings from left to right.  "9" is a greater than "100000" because looking at them from left to right, the first character of "9" is greater than the first character of "1".

In the table 1a-FillDetails the following fields are in place as follows:
FillDate = Date of the fill usually todays date formatted dd-mmm-yyyy  Date/Time
RecYear = Record year formatted as "yyyy"-"mm" is a ShortText
SeriesNUm = the next number in the series based on RecYear is a shortText
All this goes into the FDRecordID formatted as 202303-1, 2, 3, etc  then 202304-1, 2, 3 etc
So if i read this correctly, the "SeriedNum" should be a number format?

Thank you

SerialNum should be a numeric if you want to retain the same VBA code.  If you want to keep SerialNum as short text, you can modify your VBA code slightly to this:

     vlast = DMax("Val(SeriesNum)", "1a-FillDetailsT", "RecYear='" & Me.RecYear.Value & "'")

The Val() function above converts any string that looks like a number into an actual number.  Therefore, DMax() will return the numerical max.

Caveat: Access defines "looks like a number" as having a numeral as the first character.  Hence, Val("2") and Val("2abcdefghij") both return the numeral 2.  But Val("a2") will return 0 instead.  Val() returns 0 for any string that doesn't "look like a number."

My suggestion is to actually change SerialNum to a numeric field.  Any field that can only be numeric (and more importantly, must be numeric in order to work, as in your case here) should be a numeric field.

Kevin; Thank you so much. I changed the field to a numeric and yes it worked and is working as intended. Hugely appreciated.