在Access VBA中，Shell函数用于执行外部程序。然而，有时在特定的数据库中，Shell命令可能失效。以下是一些解决方法：

1. 确保Shell函数的参数正确无误。确保传递给Shell函数的路径和参数是正确的。例如，确保传递给Shell函数的路径是有效的可执行文件路径。

2. 检查安全设置。在某些情况下，Access可能会限制执行外部程序的权限。在Access中，打开文件->选项->当前数据库->启用宏时，确保“启用所有宏”选项已选中。

3. 使用API函数代替Shell函数。有时候，使用API函数可以解决Shell命令失效的问题。例如，使用CreateProcess函数来执行外部程序。下面是一个使用API函数来执行外部程序的示例代码：

Declare Function CreateProcess Lib "kernel32" Alias "CreateProcessA" (ByVal lpApplicationName As String, ByVal lpCommandLine As String, ByVal lpProcessAttributes As Long, ByVal lpThreadAttributes As Long, ByVal bInheritHandles As Long, ByVal dwCreationFlags As Long, ByVal lpEnvironment As Long, ByVal lpCurrentDirectory As String, lpStartupInfo As STARTUPINFO, lpProcessInformation As PROCESS_INFORMATION) As Long

Type STARTUPINFO
    cb As Long
    lpReserved As String
    lpDesktop As String
    lpTitle As String
    dwX As Long
    dwY As Long
    dwXSize As Long
    dwYSize As Long
    dwXCountChars As Long
    dwYCountChars As Long
    dwFillAttribute As Long
    dwFlags As Long
    wShowWindow As Integer
    cbReserved2 As Integer
    lpReserved2 As Long
    hStdInput As Long
    hStdOutput As Long
    hStdError As Long
End Type

Type PROCESS_INFORMATION
    hProcess As Long
    hThread As Long
    dwProcessID As Long
    dwThreadID As Long
End Type

Sub RunExternalProgram()
    Dim proc As PROCESS_INFORMATION
    Dim start As STARTUPINFO
    Dim retVal As Integer
    
    '设置启动信息
    start.cb = Len(start)
    
    '启动外部程序
    retVal = CreateProcess("C:\Path\to\Program.exe", "", 0, 0, 1, 0, 0, "", start, proc)
    
    If retVal <> 0 Then
        '程序已成功启动
        '可以根据需要进行一些操作
    Else
        '启动程序失败
        MsgBox "无法启动外部程序"
    End If
End Sub

上述代码使用CreateProcess函数来执行外部程序。您需要将程序的路径替换为实际的可执行文件路径。

这些是解决在特定数据库中Shell命令失效的一些常见方法。您可以根据具体情况选择适合您的解决方法。